Sunday, October 23, 2011

Treating Cats with Hyperthyroidism: Antithyroid Drugs

In cats, hyper­thyroidism can be treated in four ways — chronic administration of an antithy­roid drug, surgical thyroidectomy, radioactive io­dine (131-I), or lifelong feeding of an ultra-low iodine diet.

The treatment of choice for an individual cat depends on several factors, including the age of the cat, presence of associated heart or kidney dis­eases or other major medical problems, availability of a skilled surgeon or radioiodine treatment facility, and owner's preference (1-4).

In this post, I’m going to discuss the use of antithyroid drugs for treating cats with hyperthyroidism. This is the most common means that veterinarians use to treat this common condition, so let’s start by discussing the pros and cons of this form of treatment.

The Advantages and Disadvantages of Antithyroid Drugs

Chronic management with antithyroid drugs is a practical treatment option for many cats with hyperthyroidism, and offers many advantages. Medical management requires no special facilities and can be prescribed by all veterinarians (1-4). These drugs cause a rapid fall in serum thyroid hormone levels (i.e., the high serum T4 normalizes within 1-3 weeks), which may be desirable in severely affected hyperthyroid cats (5,6).

Anesthesia is avoided, as are the surgical complications associated with thyroidectomy (I’ll be discussing surgical treatment in my next post). In contrast to surgery or radioiodine treatment, hospitalization is not required with medical treatment. Finally, the initial, upfront costs of antithyroid drugs is much less than with either surgical or radioactive iodine treatment.

Long-term medial management also has many disadvantages. This form of treatment is not curative, is highly dependent on owner and cat compliance, and requires regular biochemical monitoring to ensure the efficacy of treatment (1-4). Side effects are common, occurring in up to 20% of cats (1-6). Even though the initial cost of medical treatment may be far less initially, the cost of ongoing monitoring over a period of months to years can exceed that of thyroidectomy or radioiodine therapy.

These antithyroid drugs also come with other drawbacks. Since they block thyroid hormone synthesis but do not destroy the cat’s thyroid tumor, these drugs never cure the hyperthyroidism and relapse will always occur if daily medication is discontinued (1-6). Most importantly, the benign thyroid tumor — which is present in all cats with hyperthyroidism (7,8) — continues to grow and, after many months, may transform from adenoma to thyroid carcinoma in some cats (9).

Long-term medical management is best reserved for cats of advanced age or for those with concurrent diseases, and for when owners refuse either surgery or radioactive iodine. In addition to long-term treatment, medical management is also advised prior to surgical thyroidectomy to decrease the metabolic and cardiac complications associated with hyperthyroidism. Short-term medical management is often recommended as trial therapy to determine the effect of restoring euthyroidism on kidney function, especially in cats with suspected chronic kidney disease (1-4,10).

Methimazole and Carbimazole: The 2 Antithyroid Drugs

The two drugs methimazole and carbimazole are commonly recommended for managing cats with hyperthyroidism (1-6). A related drug, propylthiouracil, often used in human medicine, is not recommended for cats because of a high incidence of serious adverse reactions —especially anemia and bleeding problems (11).

Methimazole blocks thyroid hormone synthesis by inhibiting thyroid peroxidase, an enzyme involved in the oxidation of iodide to iodine, incorporation of iodine into thyroglobulin, and coupling of tyrosine residues to form T4 and T3 (12). Methimazole does not block the release of preformed thyroid hormone, so there is a delay of 1 to 3 weeks before serum T4 concentrations return to normal after initializing therapy (5, 6).

Carbimazole is a pro-drug of methimazole. That means that after oral administration of carbimazole, it is almost immediately converted to methimazole by the cat (13). So basically, it turns out that carbimazole and methimazole end up being the same drug (1-4).

Different Drug Formulations of Methimazole & Carbimazole

Methimazole Tablets
Methimazole is specifically licensed for treatment of feline hyperthyroidism both in the USA and Europe as 2.5- and 5-mg tablets (Felimazole, Dechra Veterinary Products). It is also available as a generic and brand name drug for human use (Tapazole). For most hyperthyroid cats, a starting dose of 1.25 mg to 2.5 mg methimazole is administered twice daily is recommended (1-4,14).

Carbimazole Tablets
Carbimazole is available for human use in many European countries (brand name, NeoMercazole), Australia and Japan (2-4). This drug is not available as a licensed drug in the USA, but it is available though compounding pharmacies.

As noted above, carbimazole exerts its antithyroid effect through immediate conversion to methimazole when administered orally (13). Serum concentrations of methimazole achieved after carbimazole administration are less than after a similar weight of methimazole such that a 5-mg dose of carbimazole is approximately equal to 3 mg of methimazole (3,6). Because of that, a starting dose of regular carbimazole of 2.5 mg to 5 mg twice daily is commonly recommended for restoring euthyroidism (2-4,6).

Carbimazole is often touted as having a lower incidence of adverse reactions such as vomiting and anorexia (6,15). This may be because it is tasteless whereas methimazole has a bitter taste (3,6). However, Felimazole, as licensed for veterinary use, is sugar-coated; provided the tablet is not crushed, the bitter taste is presumably avoided.

Carbimazole Tablets (Controlled-Release)
A controlled-release formulation of carbimazole (Vidalta, Intervet Schering Plough) is licensed for cats in Europe for once daily administration (16,17). This formulation is not available in the USA.

Administration of this drug with food significantly enhances its absorption (16). The starting dose for controlled release carbimazole is 15 mg administered once daily. In cats with mild hyperthyroidism (total T4 concentration <100 nmol/L or < 8 μg/dl), a 10 mg once daily is recommended (2-4,16).

Transdermal Antithyroid Drugs
Carbimazole and methimazole can be reformulated by a veterinary compounding pharmacy and applied to the non-haired inner portion of a cat’s pinnae (ear lobe) for transdermal administration (18-20). Such custom formulation increases expense of therapy and the stability of the product can never be guaranteed.  To prevent absorption of the drug through one's own skin, it is best to wear gloves or a finger cot for application, and wash your hands afterwards.

Both antithyroid drugs are generally effective in cats when administered at a dose of 1.25 mg to 2.5 mg twice daily transdermally (2-4).  One advantage of using a compounded formulation of methimazole (or carbimazole) over the 2.5- or 5-mg tablets is that it is easier to make smaller or finer dose adjustments.

Transdermal administration is associated with fewer gastrointestinal side effects than the oral route (19,21,22), but some cats resent manipulation of their ears and crusting can occur between doses leading to erythema. These problems can usually be prevented by removing any crusted material and cleaning the ear flap prior to administration.

Monitoring of Hyperthyroid Cats on Antithyroid Drug Treatment

Initial doses of the antithyroid drug vary depending on the cat’s pretreatment serum T4 value and goiter size (i.e., size of the thyroid tumor). In general, however, most cats are started on 1.25-2.5 mg of methimazole or 2.5-5 mg of carbimazole, both administered twice daily (1-4,21-23).

Initial Monitoring
Initially, cats should be reassessed after 2 to 3 weeks and a serum total T4 concentration measured. When monitoring, time of serum T4 sampling in relation to the administration of the antithyroid drug is not overly important (1-4,24). The goal of medical therapy is to maintain total T4 concentrations within the lower half of the reference range (1-4). Low serum T4 values should be avoided, however, because it has been shown that chronic hypothyroidism is deleterious to kidney function and may worsen already present chronic kidney disease (25,26).

If hyperthyroidism persists during antithyroid drug treatment, lack of owner or cat compliance should always first be eliminated as a reason for the failure of therapy. If the serum T4 concentrations remain high with proper treatment, however, the daily dose of methimazole or carbimazole can be increased in 2.5-mg increments, reassessing the cat again in 3 to 4 weeks (1-4).

Long-Term Monitoring and Treatment
For chronic management (once euthyroidism has been achieved), the daily antithyroid drug dosage is adjusted to the lowest possible dose that effectively maintains euthyroidism. Once the dosage has stabilized, the cat should be monitored every 3 to 6 months and as needed clinically. At time of each of these rechecks, a complete physical examination should be perform together with determination of a complete blood count, serum chemistry profile, and serum T4 concentration.

Relapses are common in cats treated with an antithyroid drug. Some cats will become more difficult to medicate over time, whereas others will need higher daily drug dosages to inhibit thyroid hormone secretion as their thyroid tumors continue to grow larger and larger (27).

It is important to keep the serum T4 concentration within the mid-normal range and not have even mildly high or high-normal values. For example, if the T4 reference range is listed as 0.8-4.0 μg/dl (10-50 nmol/L), my goal is maintain the T4 values between 1.5-2.5 μg/dl (20-32 nmol/L). Recent research indicates that hyperthyroidism may contribute to the development or progression of chronic renal disease in cats (28-30).  Leaving a hyperthyroid cat untreated (or poorly regulated with methimazole or carbimazole) may therefore be detrimental to long-term kidney function and is never recommended.

During long-term treatment, it is again important to avoid inducing hypothyroidism, which may be deleterious to the cat’s kidney function (25,26). If hypothyroidism is suspected (which can develop even if the T4 is low-normal), a complete thyroid panel is recommended, including determination of the serum concentrations of total T4, free T4, T3, and TSH (see my previous blog posts on diagnostic testing for more information about these tests). The findings of low serum free T4 with high TSH concentrations is diagnostic for iatrogenic hypothyroidism; in those cats, the daily dose of methimazole  should be decreased.

Because antithyroid medications have no effect on the underlying lesion, the thyroid nodules continue to grow larger and larger over time. This may necessitate an increased daily dose with time. In some cats, large enough dosages can no longer be administered to control the hyperthyroidism and surgery or radioiodine is needed to control the hyperthyroidism (27).

Side Effects & Adverse Reactions

Most clinical adverse reactions occur within the first 3 months of therapy (1-4). Mild clinical side effects of vomiting, anorexia, or depression occur in approximately 10-15% of cats, usually within the first 3 weeks of therapy (4,5). In most cats, these reactions are transient and do not require permanent drug withdrawal.

Mild Side Effects
Figure 1: Facial excoriations
due to methimazole
Early in the course of drug therapy, mild and transient hematological abnormalities, including leucopenia (low total white blood cell count), lymphocytosis (high lymphocyte count), or eosinophilia (high eosinophil count) develop in up to 15% of cats without any apparent clinical effect (1-6).

Self-induced excoriations of the head and neck (from scratching) occasionally develop, usually within the first 6 weeks of therapy (see Figure 1).

Less commonly, generalized enlargement of lymph nodes may develop during drug treatment (31). If either of these adverse effects occur, the drug must be stopped and another form of treatment given.

Life Threatening Side Effects
Figure 2: Bleeding from gums due
to methimazole
More serious hematological complications occur in less than 5% of cats and include a severe lowering of the white blood cell count (agranulocytosis) or platelet count (thrombocytopenia).  Liver dysfunction, characterized by marked increases in all hepatic enzymes, develops in less than 2% of cats (1-5).

Rarely, cats may also develop a severe bleeding tendency during drug treatment (see Figure 2) (5,32). All of these adverse effects are reversible upon discontinuation of the medication.

The Bottom Line

Short-term or chronic management with antithyroid drugs provides a useful treatment option for many cats with hyperthyroidism. However, this treatment does not cure the disease and requires daily medication for the rest of the cat's life.  Like any prescription drug, methimazole or carbimazole can produce adverse side effects, which may be life-threatening is some cats. For all of these reasons, antithyroid drugs will never be the "treatment of choice" for all cats with hyperthyroidism.

  1. Peterson ME: Hyperthyroidism in cats. In: Melian C (ed): Manual de Endocrinología en Pequeños Animales (Manual of Small Animal Endocrinology). Barcelona, Multimedica, 2008; 127-168.
  2. Baral R, Peterson ME. Thyroid diseases. In: Little, S. (ed), The Cat: Clinical Medicine and Management. Philadelphia, Elsevier Saunders 2012; in press.
  3. Mooney CT, Peterson ME: Feline hyperthyroidism, In: Mooney C.T., Peterson M.E. (eds), Manual of Canine and Feline Endocrinology (Fourth Ed), Quedgeley, Gloucester, British Small Animal Veterinary Association, 2012; in press.
  4. Peterson ME: Hyperthyroidism in cats, In: Rand, J (ed), Clinical Endocrinology of Companion Animals. New York, Wiley-Blackwell, 2012; in press.
  5. Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine 1988;2:150–157. 
  6. Mooney CT, Thoday KL, Doxey DL. Carbimazole therapy of feline hyperthyroidism. Journal of Small Animal Practice 1992;33:228–235. 
  7. Gerber H, Peter H, Ferguson DC, et al. Etiopathology of feline toxic nodular goiter. Veterinary Clinics of North America Small Animal Practice 1994;24:541-565.
  8. Peterson ME, Ward CR. Etiopathologic findings of hyperthyroidism in cats. Veterinary Clinics of North America Small Animal Practice 2007;37:633-645.
  9. Hibbert A, Gruffydd-Jones T, Barrett EL, et al. Feline thyroid carcinoma: diagnosis and response to high-dose radioactive iodine treatment. Journal of Feline Medicine and Surgery 2009;11:116-124.
  10. Becker TJ, Graves TK, Kruger JM, et al. Effects of methimazole on renal function in cats with hyperthyroidism. Journal of the American Animal Hospital Association 2000;36:215–223. 
  11. Peterson ME, Hurvitz AI, Leib MS, Cavanagh PG, Dutton RE. Propylthiouracil-associated hemolytic anemia, thrombocytopenia, and antinuclear antibodies in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1984;184:806-808. 
  12. Cooper DS. Antithyroid drugs. New England Journal of Medicine 2005;352:905-917.
  13. Peterson ME, Aucoin DP. Comparison of the disposition of carbimazole and methimazole in clinically normal cats. Research in Veterinary Science 1993;54:351–355. 
  14. Plumb DC. Plumb's Veterinary Drug Handbook (7th Ed). PharmaVet Inc, Stockholm, Wisconsin 2011.
  15. Bucknell DG. Feline hyperthyroidism: spectrum of clinical presentions and response to carbimazole therapy. Australian Veterinary Journal 2000;78:462-465. 
  16. Frénais R, Burgaud S, Horspool LJ. Pharmacokinetics of controlled-release carbimazole tablets support once daily dosing in cats. Journal of Veterinary Pharmacology and Therapeutics 2008;31:213-219.
  17. Frenais R, Rosenberg D, Burgaud S, et al. Clinical efficacy and safety of a once-daily formulation of carbimazole in cats with hyperthyroidism. Journal of Small Animal Practice 2009;50:510-515. 
  18. Hoffman S, Yoder A, Trepanier L. Bioavailability of transdermal methimazole in a pluronic lecithin organogel (PLO) in healthy cats. Journal of Veterinary Pharmacology and Therapeutics  2002;25:189-193. 
  19. Hoffman SB, Marks SL, Taboada J et al. Transdermal methimazole treatment in cats with hyperthyroidism. Journal of Feline Medicine and Surgery 2003;5:77–82. 
  20. Buijtels JJ, Kurvers IA, Galac S et al. Transdermal carbimazole for the treatment of feline hyperthyroidism, Tijdschrift voor Diergeneeskunde 2006;131:478-482. 
  21. Trepanier LA. Pharmacologic management of feline hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 2007;37:775-788. 
  22. Trepanier LA. Medical management of hyperthyroidism. Clinical Techniques in Small Animal Practice 2006;21:22-28. 
  23. Trepanier LA, Hoffman SB, Knoll M, et al. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association 2003;222:954–958. 
  24. Rutland BE, Nachreiner RF, Kruger JM. Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats. Journal of Veterinary Internal Medicine 2009;23:1025-1030. 
  25. Williams TL, Peak KJ, Brodbelt D, et al. Survival and the development of azotemia after treatment of hyperthyroid cats. Journal of Veterinary Internal Medicine 2010;24:863-869. 
  26. Williams T, Elliott J, Syme H. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. Journal of Veterinary Internal Medicine 2010;24:1086-1092. 
  27. Peterson ME. Treatment of severe, unresponsive, or recurrent hyperthyroidism in cats. Proceedings of the 2011 American College of Veterinary Internal Medicine Forum. 2011;104-106.
  28. Lapointe C, Bélanger MC, Dunn M, et al. N-acetyl-beta-D-glucosaminidase index as an early biomarker for chronic kidney disease in cats with hyperthyroidism.  Journal of Veterinary Internal Medicine 2008;22:1103-1110. 
  29. van Hoek I, Lefebvre HP, Peremans K, et al. Short- and long-term follow-up of glomerular and tubular renal markers of kidney function in hyperthyroid cats after treatment with radioiodine. Domestic Animal Endocrinology 2009;36:45-56.  
  30. van Hoek I, Meyer E, Duchateau L, et al. Retinol-binding protein in serum and urine of hyperthyroid cats before and after treatment with radioiodine. Journal of Veterinary Internal Medicine 2009;23:1031-1037. 
  31. Niessen SJ, Voyce MJ, de Villiers L, et al. Generalised lymphadenomegaly associated with methimazole treatment in a hyperthyroid cat. Journal of Small Animal Practice 2007;48:165-168. 
  32. Randolph JF, DeMarco J, Center SA, et al. Prothrombin, activated partial thromboplastin, and proteins induced by vitamin K absence or antagonists clotting times in 20 hyperthyroid cats before and after methimazole treatment. Journal of Veterinary Internal Medicine 2000;14:56-59. 


Anonymous said...

I agree completely!

jennyk said...

Dr Peterson, you say that the starting dose for 'regular' carbimazole is 5 to 10mg a day but for slow-release carbimazole it is 15mg a day. Is that because not all the slow-release form is released and absorbed by the cat or is there another reason?

Dr. Mark E. Peterson said...

The absorption kinetics of the slow-release carbimazole preparation is different from the "regular" carbimazole preparation. As it states in the name, the slow release formulation released more slowly into the bloodstream. And that affects the dosage needed to block T4 secretion by the cats' thyroid tumors.

jennyk said...

Thank you!

K said...

Can you please share your thoughts on whether topical carbimazole might be better tolerated than topical methimazole. I have sibling cats who are hyperthyroid. They have side effects from Methimazole at low doses (1mg/1.5mg). I am in the process of obtaining a more dilute compounded gel so I can start again in a few days with a .5mg dose. But if you think I might have better luck with carbimazole, I will try to locate some.

Many thanks for your helpful blog =)

Dr. Mark E. Peterson said...

If your cats are showing Gi effects to the transdermal methimazole, it's unlikely that a switch to carbimazole would help. Once absorbed, carbimazole must be converted to methimazole in the liver to work, so basically, it's the same drug.

When given orally, methimazole has a bitter taste, whereas carbimazole is fairly tasteless. I believe that's why we see less GI side effect with oral carbimazole. But once we by-pass the GIT, the taste of the drugs isn't a factor.

K said...

Thanks for your response. I was hoping it would be otherwise. I will still try carbimazole, because the cats want clean their ears after I apply the Methimazole. At the very least, switching to carbimazole compounded to the lowest available should help me start with a very low dose and gradually increase. Thanks again for your help. Keep up the good work. =)

Dr. Mark E. Peterson said...

Let me just say this to clear — I have not ever used transdermal carbimazole so I can't say that it won't work better than the transdermal methimazole. But if it is better tolerated, I would be a bit surprised ... keep me posted.

Dawn Eaton said...

Hi. I have a 13 year old cat and she was just diagnosed with hyperthyroidism, and her count or level was/is 16. I understand from reading on your website, that this is very bad. The vet has put her on tapazole 5mg 3 times daily. Is that a normal amount? Also, she has only been on it for a week and she is exhibiting signs of that facial itching, not good, especially her ears. I think I am going to go the iodine route, I hope her level is not too high for that? Thank you for any insight you can give.

Dr. Mark E. Peterson said...

A serum T4 concentration of 16 µg/dl is higher than the average T4, but the value is not extremely high. Most cats do not need 15 mg of methimazole per day, but the dosage needed is quite variable. Most cats with a T4 of 16 µ/dl can be controlled on daily doses of 5-7.5 mg.

The itching is an allergic reaction. You can lower the dose to see if that helps, but in most cats that side effect is not dose related.

Radioiodine should be a good treatment for your cat, especially if all of her other body functions are good (no kidney disease, etc).

Jenn K said...

Hi Dr. Peterson! I came upon your site just by happenstance. My 9 yr. old male has Hyperthyroidism and has had it since probably October 2013. I put him on Methimizole 1/2 bid. He was doing great on this dose and his values went from 7.5 down to 3 or 4 (or so). He unfortunately now has liver failure due to the meds. This makes me VERY angry and frustrated, and I have tried homeopathy for him, but in reading everything, the tumor can grow. (I guess I had forgotten about this).

Anyway, I am considering radioactive iodine therapy. Should I do this w/my cat that has liver failure? He doesn't look jaundice any longer as I am using homeopathy on him for liver failure, but what are your thoughts? Cost, how long do the results last? Can the tumor grow back?

Thank you SO much for any input you can provide!!

Dr. Mark E. Peterson said...

The liver failure should be reversible once the methimazole is stopped.

If severe liver failure continues, then you need an ultrasound and liver biopsy to diagnosed the underlying problem and determine the best course of treatment.

Jenn K said...

Thanks so much for your response. I just found out that the course of tx here is $1275!! Unfortunately, I cannot afford that... :-(

Cristina Samaniego said...

Dr. Peterson,

My 13 yr. old cat was just diagnosed with hyperthyroidism-value of 13- and was prescribed 5mg/once daily of Felimazole. My concern is I am leaving town early next week and will be gone for 9 days. Though I have someone coming to check on her, it won't be daily, so do I start her on the regimen now, knowing she won't receive it 4-5 days while I'm gone or wait until I get back? I haven't started her on the Felimazole yet since I just picked it up this evening. I'm worried about both the side effects and not being here to catch them and also any rebound effects from starting, stopping, and then starting the meds again. I feel the latter is more dangerous, but with her high values, I'm also hesitant to hold off the medication.

Any insight to pros and cons would be greatly appreciated.

Dr. Mark E. Peterson said...

Methimazole (Felimazole) takes a few days to start to inhibit thyroid hormone secretion from your cat's thyroid tumor. Then if you stop giving the drug, the effects of the drug quickly wear off, and the serum thyroid values will rise again into the hyperthyroid range.

I don't see any reason or advantage of starting the Felimazole until you get back from your trip. If you travel frequently, you should consider definitive treatment for the thyroid tumor so you don't have to worry about this.

Remember that all of these cats have a thyroid tumor and most of these thyroid tumors will continue to grow slowly with time. At 13-years of age, your cat hopefully will live a long time, giving the thyroid tumor a long time to grow.

Jo said...

Dr. Peterson,
My 19 yr old cat with hyperT (for a 2nd time following I131 7 yrs ago!), as well as Stage 3 CKD, has been on 1.25mg Felimazole since last Oct. His Jan T4 & FT4 levels were a little less than mid-normal range, so his hyperT appeared to be well controlled.

We retested this month & although his T4 is the same as in Jan (1.8), his FT4 has jumped to 74! My vet & I were very surprised to see such a significant difference in the two numbers! The only difference in the Jan & April tests is that blood was drawn about 2.5hrs after his morning Felimazole dose in Jan, whereas it was drawn about 7 hrs after his am Felimazole dose for the April test.

Is this therefore evidence that the 1.25mg dose was, in fact, NOT controlling hormone production for an adequate length of time before the next dose as the Jan tests led us to believe? Is it, therefore, better to draw blood several hrs after a Felimazole dose to accurately determine if hormone production is being adequately controlled on a consistent, ongoing basis?

We are increasing my cat's dose to 2.5mg BID as a result of the latest test results. Given the dramatic differences between the T4 & FT4 levels, I hope most vets are running FT4 tests along with T4 when monitoring cats on longterm drug treatment!

Thank you for all you do, Dr. Peterson!


Dr. Mark E. Peterson said...

These last results do not make a lot of sense, especially if you thought your cat is doing well. I suspect that the T4 or FT4 is wrong, but I can't tell which one. It is possible that the CKD is suppressing the total T4 more than the free T4 but if that's the case, I'd expect your cat to be clinically sick (not eating well, etc).

If possible, I'd like to do a whole panel, including a total T4, T3, free T4 (by dialysis) and TSH. Then we can look at the whole panel to determine what dose adjustments to make.

I would not have doubled the methimazole dose based on a T4 of 1.8. This will likely lead to hypothyroidism and could lead to worsening of the renal disease.

Talk to your vet about all of this. You should recheck within a week of any dose change.

Dr. Mark E. Peterson said...

These last results do not make a lot of sense, especially if you thought your cat is doing well. I suspect that the T4 or FT4 is wrong, but I can't tell which one. It is possible that the CKD is suppressing the total T4 more than the free T4 but if that's the case, I'd expect your cat to be clinically sick (not eating well, etc).

If possible, I'd like to do a whole panel, including a total T4, T3, free T4 (by dialysis) and TSH. Then we can look at the whole panel to determine what dose adjustments to make.

I would not have doubled the methimazole dose based on a T4 of 1.8. This will likely lead to hypothyroidism and could lead to worsening of the renal disease.

Talk to your vet about all of this. You should recheck within a week of any dose change.

LynnT said...

Dr. Peterson,

My 18 yr old cat was diagnosed in mid-February with hyperthyroidism & began 5 mg Tapazole immediately.

A week later he had surgery to remove a tumor from his upper left left. They shaved a small area on his neck & both legs. I assume blood work & IV's.

Shortly thereafter he began scratching his neck on the left side where he had been shaved.

His T-4 was retested in March & was in normal range.

Just recently his scratching has become a problem, a lot of fur is now missing on the left side of his neck & behind his left ear. He also had some small sores on the neck.

The vet's office gave me Derma-Vet ointment, saying that it was due to shaver burn. The ointment seemed to make matters worse, I was finding hair all over the house & the bald area on his neck became larger. I stopped the ointment yesterday.

I see that they can have this type of allergic reaction to Tapazole.

What is the protocol when this happens?

Am I correct in assuming that a lower dose does will not resolve this?

Any insight is appreciated.

Thank you

Dr. Mark E. Peterson said...

It certainly sounds like a reaction from the methimazole. The only way to know for certain is to stop administering the drug for at least 2 weeks - if the scratching and sores improve, then it is likely that it's due to the methimazole.

In that case, you could always try giving the methimazole again to see if the sores recur; if they do, another treatment has to be used. This side effect is not dose related so a lower dose will not help.

LynnT said...

Dr Peterson,

Thank you for your fast reply.

I have a couple more questions, How soon after starting Tapazole does a allergic reaction typically appear?

How soon after stopping the Tapazole should the scratching stop?

Thank you once again for your help.

Dr. Mark E. Peterson said...

Most cats that develop an allergic reaction will do so within the first 6 weeks, but this can develop after many weeks in a few cats.

If an allergic reaction develops, most cats will decrease the amount of scratching within 2-3 days but it may take 2-4 weeks for the lesions to heal. Generally, a notable improvement is observed within 7-10 days.

cat lady said...

I have a 14 year old cat whose been treated for a couple of years with methimazole 2.5 mg bid. She recently has lost weight. Is there another medication? I know about the other treatment options, but not able financially. Have you heard of iopanoic acid?

Dr. Mark E. Peterson said...

see this blog post:

Janice Cox said...

DR Peterson,my 13 year old cat Rosie has today been diagnosed from blood test with hyperthyroidism,she has been given Vidalta a day, her T4 level is 228' the vet says this is the highest level he has seen,he says that she needs to get her weight up and feeling well before he could consider removal of the thyroid. Her kidneys thank goodness showed no signs of disease at this stage,however he said that he wouldn't rule out minor underlying kidney problems but this won't become clear until she has been on Vidalta for a while. My reason for writing to you is that I am worried for her, she is such a lovely cat,always been very loving a calm and I want to do the right and best thing for her. I would welcome your comments and advice as this is all new to me, I have read loads of information online and find your blogs of most help. I am not sure how to go forward with her diet, my gut instinct based on what I have read (been online most of night..couldn't sleep as so worried) is to go back to real foods for all 3 of my cats, so have cooked chicken in water and put this in with one sachet of wet food between 3... all eaten. I have been slightly put off feeding kibble as have read about high carbs.and not enough enough protein. I would very much welcome any nutritional advice please. Jan :)

Dr. Mark E. Peterson said...

You want to feed your cat a good balanced diet that is relatively low in carbohydrates and relatively high in protein. That said, you can't just add cooked chicken to the diet - that will result in dietary mineral imbalances.

I'd recommend that you consult with a veterinary nutritionist to get some help with this. Dr. Lisa Pierson's site ( may be helpful for you, and she may be able to consult with you to help.

Pet Vet said...

Dr. Peterson,
I have a patient who was diagnosed hyperthyroid and diabetic by his previous vet. He was prescribed Methimazole 5mg BID and glipazide (owner never showed me the dose) and he has been on both medications for more than 3 years. Sam is a 12 yo MN DSH who is not quite himself lately.

Owner says he is always hungry but she is not sure she feeds him enough, PU/PD, lethargic, no weight loss.

We ran a fructosamine level which was 636, blood glucose was 336 mg/dL, Total T4 was 1.2 ug/dL ( ref range 0.5-3ug/dL). ALT 109, Globulin 3.9, and lymphopenia (14) are the only other abnormal results.

I wanted to make sure his hyperthyroid was properly managed before treating with insulin but I am not sure how to interpret his test result. His owner works nights and she gives him his Felimazole at midnight. She was going to bring him to me at 10am the next morning for his blood draw but did not actually come in until 2:30pm which was 14.5 hrs since his last dose. I suspect if the level was taken 6 hrs post pill his T4 would have been much higher and a dosage reduction is called for but I could use your expert advice on this matter.

Thank you kindly :)

Dr. Mark E. Peterson said...

Sounds like the hyperthyroidism is under good control. If the T4 was high or high-normal at this testing time, I'd be more concerned.

I would indeed recommend starting insulin therapy, as you plan to do. THe glipizide obviously isn't working!

JoJo said...

Hi, Dr. Peterson! Have questions re my 19 yr old hyperT cat, Lucky who you have previously helped me with. Was DXd last Oct--2nd bout of HyperT. First time was 2006 for which he received I131. Too old & concurrent CKD now; BUN is 67, creat is 4.3.

Felimazole was started in Oct at 1.25mg BID. Recent T4 was 3.1. Wish I could increase dose as he has recently become restless, doing some howling & has increased appetite (which is actually good as he is underweight). But due to CKD & the following 2 problems, I fear a dose increase would cause even greater issues.

--1st problem: In the last few mos, his head has begun itching rather significantly above the eyes. Can't scratch well with hind feet due to arthritis, but rubs the areas a lot with his paws. Drives him nuts. Assuming this is Felimazole side effect. Do you think a daily dose of some sort of antihistamine, or perhaps some regimen of prednisolone (but not daily) would help calm the itching enough to allow him to continue the med? If we stop it, he's screwed! No other treatment options for him! Could really use your suggestions & dosing instructions!!

--2nd problem: Since April, his ALT has climbed from 38 to 59 to 74, and AST from 19 to 29 to 64. Again, afraid this is Felimazole caused! Have started 50mg milk thistle SID, hoping to titrate up to 150mg if no vomiting. Don't know what else to do. Should I be real concerned?

Thanks so much!
Jo Gunter

Dr. Mark E. Peterson said...

I would not worry about the liver enzyme elevations - they are mild and not clinically significant.

I don't know of anything to give to stop the pruritis that is secondary to methimazole. Talk to your vet about trying antihistamines or a low dose of a steroid and see if that help.

JoJo said...

WILL DO! Thank you, thank you, thank you for your reply!!!


Debi Hart said...

Dr. Peterson. I have 12 years old female cat who has been diagnosed with 'slightly" elevated thyroid.Since I knew nothing about it I have never asked of any numbers/level that I just saw on so many comments. However, I was told all her blood test looks great except of the "elevated level'"of thyroid. Since she is not taking any oral meds without spitting it out, biting and scratching, her doctor prescribed methimazole ointments at compound pharmacy at 1.0mg dose,the lowest one. However,my cat started vomiting so bad just 5 days after.But at the same time she had another undergoing issue when we found she with eating an artificial plant that was blocking her stomach. So,with these two ongoing things at the same time and without knowing exactly what have caused her severe vomiting I stopped her Methimazole use until her stomach was normalized.I didn't know if her vomiting was caused by eating the plant or either it was due to Methimazole ear gel. Well...she was doing fine all last month without methimazole but I knew I can't ignore her thyroid and still have to re-start Methimazole gel again.Her vet told me to begin administering just the tiniest part of her prescribed dose to rapidly increasing it.So, for the past 5 days I started giving her 1/4 of her needed 1.0mg dose twice a day. It was really low but I thought to start her at least on that. Well,yesterday morning she has made an attempt to contract her throat and vomit some clear stuff.It wasn't severe or as bad as a non-stop throwing up as from her first start on Methimazole gel.However, later day she developed some weird noise within her breathing and a terrible growling in her stomach. It is initially, just happened so suddenly and lasted all night.I couldn't link some of that breathing noise to her stomach growling somehow and thought she got pneumonia that was maybe affecting her stomach too.I took her to ER. X-ray showed all of her GIT being filled with gas, but no other signs of anything else,therefore,the emergency vet wasn't quite sure about the final diagnosis but, suggested "PERHAPS", a side effect to methimazole ear gel.The vet said that the gel might be compromising her GIT as a side effect. They offered to me a PRESCRIPTION diet called Y/D But i hava a huge problem with this diet for her because of a few obstacles: I live in a small,one-story ranch with no basement and have 3 other cats too with one being 12 weeks old kitty. So I usually leave out their dry food (grain-free by Blue) and kitty's food separate in his bathroom for now. My female with thyroid problem is the one who is very picky eater and barely eats anything but Blue brand, and rather eats tiny portions few times a day than 2-3 big meals.That puts me in a position that:Firstly, she won't be touching Y/D food and secondly: even she'd be eating it,I can't leave her prescribed food for the entire day with other cats while me being at work, nor I can leave their food around her.Sorry for such long details story but it leads me to a question: Should I still try to keep her on Methimazole gel while with her adverse reaction to it(throwing up and not eating well) or should I completely stop this med? And also, how long the adverse reaction/side effects stay in cats system from Methimazole? Can it go away later while she'll get used to it or not? By short:is this reaction a temporary reaction or it will get worse even on very low dose? And also, another medication-the carbimazole,is it a better gel option than methimazole? Or it doesn't matter? Thanks a lot!

Dr. Mark E. Peterson said...

Way to many questions for me to answer here. You need to sit down and talk to your vet and come up with a plan that will work for you and your cat. Good luck.

The Rah said...

Hi Dr. Peterson, I am a big fan of your work and I thank you so much for what you do for our little friends.
I have a cat, Timon, who is 17.5 yrs and was recently prescribed transdermal methimazole as his Free T4 result was 41.3 pmol/L. I would like to be really certain that he needs this medication before i give it to him. My main concern are his kidneys, because the urea (bun) tested for 22.9 mmol/L and in July of this year it was only at 14. He is also urinating much more than usual. Also, his creatinine level is 221 umol/L. His habits are sometimes normal but sometimes fall in the hyperthyroid category which further confuses me: he gorges food (and vomits as a result) but only with certain soft foods, which we've successfully curbed by giving him smaller portions but even the smaller portions he sometimes inhales but other times he'll leave half of it and eat the rest later; is sometimes confused/anxious/restless but this only happens in the morning for less than an hour and then he is sleeping the majority of the day until dinner time. Lastly, when he was weighed in July he lost about 6 lbs since last year, and lost almost another pound by this September. I figured the lost pound was due to the vomitting since he began the gorging habit when we put him on soft food after we realized how much weight he lost; right now he is 11.7 lbs. Anyway do you think the medication is safe and warranted? My vet says he wants to see him gain weight for now. I am just worried of what the meds will do to his kidneys. Thanks, Sarah

Dr. Mark E. Peterson said...

Well, first of all, your cat definitely has kidney disease based on those values. Recommended workup should include a urinalysis and culture, urine protein measurement, and blood pressure measurement. If hypertension, proteinuria, or infection is found, those conditions should be managed.

Free T4 can be falsely elevated in cats with illness (see my other posts), so we would NEVER diagnosis hyperthyroidism on the basis of that alone. In my lab, the reference interval goes up to 50 pmol/L, so your cats value isn't that high,

About 95% of cat have a high total T4, which is a better test. Can your vet palpate a thyroid nodule? If not, I'd talk to them again or get another opinion. The weight loss and vomiting could be related to the kidney problems.

The Rah said...

Thanks so much for your reply. I will talk to my vet again (he did feel a nodule but it was small) and get a second opinion if necessary. Thanks again, Sarah

Golden Gecko Textbook Buyers, Inc. said...

Aloha Dr. Peterson,

My cat, age unknown (guess 16 to 18 yrs old) has just been diagnosed with HT. T4 is an 8. We live in Hawaii and unfortunately no one does the Radio Iodine treatment here. With the quarantine issue here in HI, coming to you in NY is a bit tough. Any ideas? Want to come out to Hawaii to treat her? Barring that, any ideas for me? We can go to Australia and New Zealand without quarantine problems, but that has its own challenges. Not happy about giving her the Methimazole for the rest of her life. Any help is appreciated.

Dr. Mark E. Peterson said...

Sorry, but even if I had time to travel to Hawaii to treat your cat, that would not be allowed (you can't transport radioactive materials without the proper licensing, etc).

I know that they do the treatment in both NZ and Australia if that is what you decide to do.

Dr. Mark E. Peterson said...

Sorry, but even if I had time to travel to Hawaii to treat your cat, that would not be allowed (you can't transport radioactive materials without the proper licensing, etc).

I know that they do the treatment in both NZ and Australia if that is what you decide to do.

Golden Gecko Textbook Buyers, Inc. said...

Aloha. Wow! That was a fast reply. Thanks so much for your time. Honestly, I would rather take her to you if possible, I will have to look into this whole quarantine issue and what it would entail to do so. I did find out that there was one Dr. who did do this procedure at VCA University Animal Hospital on Oahu, but he retired due to health reasons. I really hope that someone will start to do it again here on the islands as that would make things so much easier. Thanks again and hope you are having a great Thanksgiving holiday weekend!

Dr. Mark E. Peterson said...

And the same to you. Good luck with your hyperthyroid cat!

CR said...

Dr. Peterson: Do you know of any specialists with feline hyperthyroidism in Madison or Milwaukee, WI?

Dr. Mark E. Peterson said...

No one specifically, but there are good vets at the University at Madison.

Shona Hammonds said...

Hello Dr Peterson, thank you for your blog, it's a mine of information.

My cat, Felix, has just been diagnosed with HyT. He's been on Felimazole 2x2.5 for nearly 3 weeks. He was tested last Wednesday and he's gone down from 90 to 50 (we're in France). He's gained a lot of weight, lying down he looks great, still a bit thin when he stands up! My question is he's sleeping a lot now and is a bit slow, especially after his medication. What are the physical signs of his having too much? He's 15, has been in hospital twice with high liver enzymes and low potassium. Last week he was 2.9 K, he's now taking 2 capsules of K for Cat/day plus probiotics for his liver. The vet had to put him out a lot for the bloodtest, he wouldn't go to sleep. He doesn't have kidney disease as far as we know yet, he had an echography and she thinks his kidneys look good for his age.

Dr. Mark E. Peterson said...

The dosage is based upon the thyroid testing, as well as routine blood values (especially kidney tests, but also electrolytes). It sounds like your veterinarian know how to monitor and what to do. Good luck!

Shona Hammonds said...

Thank you! He seems a lot better now. I think in our bad luck (we had to go to emergency on a Sunday), we had a lot of luck, because we found an amazing clinic with first rate clinicians, and support staff. I would trust anyone there. Also he now has two vets and either one knows him well and is brilliant.

The isotope treatment seems amazing. Unfortunately you have to go to Belgium for it.

Interestingly my vet said she didn't have anything like 20% allergies to Felimazole. It seems French cats are genetically suited to Felimazole! The difference when you read material in French and English is marked.French cats love Felimazole!

Ren said...

Hi there, Dr Peterson. I am hoping you could give me a little insight. I've been treating my cat's thyroid condition (hyperthyroid) with the ear gel, however in doing so, I am concerned that this may be a major contributing factor to my own hair inexplicably falling out. My hair loss has coincided within a few weeks of starting my cat's medication. For a couple of months we switched compounds (from gel to creme) and within a couple of weeks my hair loss settled right down. Unfortunately the new compound was not as effective for my cat so we switched back and within a fortnight, my hair has started falling out again. I have been investigating this issue with my own doctor, but do you know of any such reactions from a possible incidental transference? I do use gloves to apply the medication but short of thoroughly washing my hands every time I touch my cat, I am not sure what to do. Cheers, Ren

Dr. Mark E. Peterson said...

I've never heard of this side-effect but the cream or gel can be absorbed from your skin. If your hair is falling out, I'd expect you to have low thyroid function, which your doctor could check.

It sounds like you need to choose another way to treat if this continues to be a problem and you are certain it's related to the methimazole.

Little_Miss said...

Hi, I just wanted to say thankyou to the doctor and all previous comments.
I have a 19 year old cat and he has been on Carbimazol for a number of years. He is slowly deteriorating with old age and I suspect he has liver problems.
We have made a family decision to not stress him out with any more tests or vet visits ( as he really gets effected by the car trip) instead we will let him eat and sleep and lap sit anytime he wants. And we know when he can't get onto our laps, his time will be near.
Till then loads of pats and cuddles.

Reading all the other comments has helped me to understand the side effects and see how other cat lovers are experiencing the hyper thyroid problem.

Thanks again, Lee (Australia)

Martha said...

I have a 18 year old male who has been on Meth for hyperthyroid and now we have discovered he has CRF or CKD very mild but starting. My vet wants me to put him HIlls K/d but he won't eat it and I don't believe in the low protein, high carbs but my concern is how can I treat my cat with anything but meds for his thyroid when he seems compromised already with his kidneys, has very few teeth and is only 8.05 lbs and used to be around 9-10lbs. He's a very picky and slow eater and I have 3 others cats. I have been feeding him whatever he will eat as long as it's low in phosphorous, low in carbs and a high end protein. I have also started giving him 100ml of fluid subQ 2xweek. Needless to say, I'm very confused and I am limited on funds but do spend money on good food. Thank you so much

Dr. Mark E. Peterson said...

I agree with your dietary management for your cat. A higher protein, lower carb canned food that is relatively low in phosphorus will help slow the progression of the kidney disease. You could also think about added a phosphate binder to the food, which will lower phosphate absorption even further. Take a look at this website (, which should help you. The kidney diets may be helpful at some point, but they won't work if your cat won't eat the food!

catnip said...

I have an 18 yo female cat that has been hyperthyroid on Methimazoel/Felimazole for years. She recently began losing a lot of weight even though thyroid levels were good. Found out she was in kidney failure. Doing better now but vet stopped her thyroid meds. I am seeing continuing weight loss even though appetite is very good now she is extremely boney. She eats Fancy Feast, and the critical care AD canned food and SQ Lactated Ringers daily. Should I resume her Felimazole with her poor kidney function?

Dr. Mark E. Peterson said...

Yes, you need to treat the thyroid condition, especially it's severe. Talk to your vet.

catnip said...

Thank you!

vildan ocalan said...

Hi doctor Peterson, my 12 yr old cat was diagnosed with HT just 3 weeks ago. After giving him methimazole 2.5mg twice a day for two weeks his side effects were severe so the vet told us to stop with the meds until they cleared up. It has been a week off the meds, the facial itching has stopped, but he is still vomiting at least a dozen times per day and barely eating. He has had blood work done twice in the last 3 weeks and his kidneys are ok. He has an appointment for the radio iodine injection in 10 days. I am concerned about the vomiting, it is so severe i might have to take him to the emergency room for dehydration. Any thoughts on the vomiting issue?

Dr. Mark E. Peterson said...

This is not from the hyperthyroidism. Talk to your vet but do not do the I-131 now until this is resolved.

Arieschick said...

My cat is 13 and just diagnosed with hyperthyroidism. Already has CKD/ renal failure. I'm worried the dose is too high for him (5mgx2).
Can I just give one pill a day for a week or two? I read it should be started low and raised gradually.

Dr. Mark E. Peterson said...

Twice a day treatment works best, but once a day is okay in some cats. I generally start most cats with CKD on a very low dose (1.25 mg once to twice a day) and adjust upward as needed. To do that, we usually need to compound the methimazole, although in some counties (eg, UK) they have 1.25 mg pills.

Jo said...

Arieschick, sells Felimazole 2.5 mg tablets. They are very small and can easily be cut in half with a sharp pill cutter!

I just don't get these vets who start cats off with such high methimazole doses, especially knowing the negative side effects this drug can have! You can always add more med, but you can't take it away once it's swallowed! Thank God for Dr. P and his generous, free guidance on the subject!

Dr. Mark E. Peterson said...

Thanks for your comment!

Diana Mirzoyan said...

Hi Dr. Peterson,

My kitty Ozzie is 13 years old; diagnosed with kidney disease from kidney stones ~1.5 years ago after a bad crash - had yo-yo'ing kidney values and even required surgeries (stent, then SUB) to help relieve obstructions but creatinine now is 2. Had hypertension last year, required high dose of amlodipine to control it, then normal blood pressure without any meds for past ~4 months (inexplicable) till had episode of collapse a few weeks ago and blood pressure was 240. So started him back on amlodipine and checked thyroid labs as part of work-up. T4 was 2-something last year; 3 weeks ago it came back as 4.0 ("gray zone") and vet was ready to treat; we asked for thyroid panel and below are results/ ranges from a week ago (T4 3.5, free T4 barely out of range). Ozzie is actually gaining back the weight he'd lost due to kidney disease, looking healthy now, so definitely no weight loss from hyperT. Since hypertension was present with T4 in 2's last year, not sure if its re-emergence has to do with CKD or hyperT; has diagnosis of mild HCM, also mildly tachycardic at vet's (200-220) though normal heart rate when I check at home with stethoscope (hr 120 yesterday). So our vet (whom I like a lot, who saved his life with SUB surgery) is recommending starting transdermal methimazole at 2.5mg once a day; husband wants to wait and re-check labs in next couple of months; I'm hoping to avoid cardiac problems from untreated hyperT but am worried about getting T4 too low/ worsening kidney function or even crash/ side effects from new med in a kitty who has chronic GI problems (IBD/ pancreatitis) but has been eating well and gaining weight and not vomiting for past couple of months. Aside from possible cardiac signs, the only other possible symptom is higher energy than in past - used to be a lazy fat kitty before all his illnesses, now sprints sometimes but not restless and sleeps a lot; grooms well and looks clean and fluffy; very quiet kitty. Do you think we should treat or wait, and is treatment with tiny dose of maybe 1.25mg a day (transdermal) a reasonable compromise? Do you think a scan would help in this situation? Is a cat with this history a decent candidate for radioactive iodine treatment? Thanks for your help.

3/16/15 results

T4 3.5 (range 0.8-4.7)

free T4 (ng/dl) 2.9 (range 0.7-2.6)

free T 4 (pmol/L) 37.3 (range 9-33.5)

- Diana

Dr. Mark E. Peterson said...

The thyroid results are all borderline. I'd either continue to monitor or do thyroid scintigraphy to diagnose. I would not start methimazole with these results.

At the next recheck, I'd do a complete panel with a TSH level. If hyperthyroid, that should be undetectable. Not diagnostic by itself but it can help...

Jennifer Smits said...

Hi, I really appreciate your blog. My cat was diagnosed with hyperthyroidism a month ago. Due to frequent travel, I really wanted to do radioactive iodine but vet and I were concerned about her kidneys. Before her diagnosis, her urine was dilute but kidney values were normal. At time of diagnosis, her urine was still dilute/but normal creatinine. A month after methimazole, the vet said her creatinine had increased "2 fold" and was just above the normal range. She does not recommend radioactive iodine and lowered the dose of methimazole.I don't want to do anything that will make my cat' kidneys worse but I've also read that this value will stabilize and won't rapidly increase once cat is no longer hyperthyroid. If her blood work was the same in another month, could she still be a candidate for radioactive iodine? Does hyperthyroid just mask kidney problems or does it actually offer some protection? My cat is 15 years old and due to weekend travel, I probably will only be able to give her methimazole once a day sat/sun and not at all on some Sundays and once on monday.

Dr. Mark E. Peterson said...

Sounds to me that methimazole will not work in your situation. We treat cats all the time that have mild CKD. Treatment doesn't cause CKD-- it's already there. By lowering the dose, you are just making the numbers look better by allowing the blood flow to the kidneys to be higher than normal. The key for successful I-131 treatment in these cats is to not overdose them and to monitor closely for hypothyroidism after radiotherapy.

Roger Aye said...


My cat discontinued Methimazole after 2 weeks due to vomiting, lethargy etc; we are now looking into radioactive iodine treatment.

Her face seemed to itch and become swollen, which I read is one possible side effect of the Methimazole. However, I stopped giving her the medication 4 days ago, and even after a steroid injection by my vet her face still appears to be swollen, and itchy as well.

Is it common for the this particular side effect to still be manifesting 4, 5 days after discontinuing Methimazole? My vet is at a loss since she's never seen this before. Any input greatly appreciated.

Dr. Mark E. Peterson said...

It can take as long as 2 weeks for the pruritus to resolve. If the swelling and itching continue after that amount of time (or at least haven't improved considerably) then your vet needs to continue the workup.

Roger Aye said...

Thank you for the quick response!

It's been 6 days since I stopped Methimazole and the facial swelling continues. She is also fairly lethargic, though eating/drinking normally (same as before being treated with Methimazole).

Should I consider treating her facial swelling/itching with prednisolone? Or leave it alone and give her time to heal?

Thank you again Dr. Peterson. I'm in NJ so it's very likely we'll schedule I-131 with your clinic in the near future.

Dr. Mark E. Peterson said...

Facial swelling is secondary to inflammation - does your cat's face look like the photos here in this post?

If the facial reaction is due to methimazole, use of prednisone does not help. If the lesion is open, the steroids might even be contraindicated. Time off the drug is the best.

Again, if this doesn't improve or worsens over the next 7-10 days, then it's likely due to something else.

Good luck!

chip glass said...

Hello Dr. Peterson
I recently had my cat treated with I 131 therapy for hyperthyroidism and he is just past the 2 week period following the treatment. I just noticed a small section of fur missing on top of his head and also above both eyes. I haven't noticed him scratching but that seems to be the case. Any thoughts or suggestions on what this would be? it wasn't like that yesterday so I want to get a head start on this as I won't be able to talk to the Vet that treated him until Monday. Thank you for your help and the help given to others as well.

Dr. Mark E. Peterson said...

This should not be a reaction or side-effect from the I-131 treatment. I don't know what's causing the hair loss but it certainly doesn't appear to be an emergency. Call your regular vet to see if they can look at your cat sooner.

Lynne Forrestal said...

My 12 yr old cat has 20.8 level, and is now on methimazole 5 mg/ml , a 1 ml dose twice daily. she weighed 7 pounds... after one week, she now weighs 6 # 6 oz. and is sleepy, and not eating. Our vet said that this is not a side effect of the meds... but she has lost more weight rapidly and without eating will lose more. she has had mild vomiting for months.
Would a reduced dosage for a period of time help? are these side effects of the meds. and should we worry? she seems so frail now. thank you

Dr. Mark E. Peterson said...

Anorexia and lethargy are relatively common side effects from the methimazole.

I'd consider stopping it completely for a couple of days to see if it's the medication. If the appetite comes back, you could then retrial her on the methimazole to see if the appetite decreases again and if so then you have to forget about using the methimazole. In addition, at 12 years of age, I'd consider a definitive treatment (such as radioiodine) since this is a progressive disease and the thyroid tumor(s) will continue to grow on the methimazole.

PK said...

Hi Dr Peterson,

My almost-16yo female cat was dx hyperT about 9 months ago, and has been on transdermal methimazole ever since. She has wavered between hyperT and hypoT due to dosing, but is now on once daily 2.5mg dosing. She also has late stage CKD, vet thinks it was hastened by her hyperT.

She has chronic facial itching which has not subsided with lowered dosing, inappetance which is getting worse (she is painfully thin at around 2.8kg/6lb on a large frame, will not groom at all, her fur has classic hyperT appearance and matting which she will not allow anyone to remove, and her weight has not increased at all since commencing treatment, due to her inappetance), and dehydration despite polydipsia, which means I have to occasionally syringe water into her mouth, and even less frequently (but mostly in summer, it's currently winter here in Perth, Western Australia) I do sub-cutaneous fluid injections, which she hates and barely tolerates.

In the last couple of days she has refused all food - even lactose-free milk (which has been our go-to to get her eating again) and today had the biggest vomit I have ever seen - it was of human proportions - and it smells vile. She has now taken to hiding (which she does when ill and distressed) and I am wondering if you have any insights. Our vet will simply give her fluids and send her home. RAI and surgery are not options as they are not available for animals here. Our ONLY treatment option is methimazole (either orally, which she will not tolerate at all, or transdermally, which seems to be giving her side effects).

If she were your cat, Doctor, what would you do?

Dr. Mark E. Peterson said...

Well, I would do radioiodine as my first choice followed by surgery. Methimazole must be stopped if you care about your cat.. no doubt about that. You comment that those 2 options are not available are puzzling to me. I-131 is certainly available in Australia, although maybe not in your neighborhood, but any small animal veterinarian should be qualified to do the surgery. Sounds like you might need a referral or a new vet!

PK said...

Thanks for your quick reply!

There is no facility for RAI in my state (WA) at all. The closest veterinary RAI operator is in NSW and there is no way Kes (my hyperT cat) would survive the trip, and no vet in WA will perform the surgery - not even the Veterinary School at Murdoch Uni (our one and only Vet School). This is what I have been told by three separate vets since Kes' diagnosis. We have been told repeatedly that the methimazole is her only available option within our state - and we cannot afford to move interstate to get her treatment (and we have no family or friends interstate we could possibly stay with to get her treated.

A new vet (it would be her fourth) is not a logical option. However, if you think she should be withdrawn from methimazole, so be it. I will speak to her vet in the next few days, and make plans for the probability of her death in the not-too-distant future :(

Thanks for your insight :)

Chandra's mom said...

Thank you for such a wonderful blog! My 14 yr old cat's senior blood profile just discovered a t4 of 9. My vet is extremely busy, and, unfortunately, has no real time to answer questions. I did read about tapazole, and the side effects scare me! so I asked him to prescribe the lowest dose for my cat, and we could always increase if necessary. He prescribed 5mg every 12 hours. Reading your blog, I dont think that is the lowest dose, so I decided to give her just half that amount, every 12 hours. It has been 4 days now, and so far so good. My question to you is: is there a chance that the 2.5mg every 12 hours will be strong enough to control her t4? The vet wants me to bring her in after 2 weeks for another blood test to see how she is handling the med. Should I keep her on the 2.5 dose, or should I go ahead and increase it to 5mg, if there is no way a 2.5 will control the t4? Her blood and urine profile was excellent, except for the high t4.
I thank you so much for your input in this!!

Dr. Mark E. Peterson said...

Many cats can be controlled with the lower dose. I would consider 5 mg twice a day to be a relatively high dose for most cats, at least at time of initial diagnosis. You can always go up if needed.

Golden Gecko Textbook Buyers, Inc. said...

Hi Chandra's Mom and Dr. Peterson. What about trying liquid carbimazole instead of the topical methimazole. We switched our cat because she was having a reaction on her ears after a bit of time. The carbimazole is tasteless and she eats it with no problem. So far so good. We get it from a compounding pharmacy here in the US. I assume you can get it as well. We do supplement that with a few herbal remedies and homeopathic remedies as well. We do not have the RAI here in Hawaii either. Just a suggestion. Good luck.

Chandra's mom said...

Thank you so much for your quick reply. I will keep her on the 2.5 dose every 12 hours and hope for the best!

Chandra's mom said...

Golden Gecko - thank you for the suggestion of the liquid carbimazole instead of tapazole. But fortunately, we can put the tapazole pill right on top of her food, and down it goes. She'll lick the bowl clean. Her nickname is "piglet", and we are so glad we can get these pills down her this way!

Dr. Mark E. Peterson said...

Chandra's mom said......Hello Dr Peterson On July 28th I wrote to you about our cat with a t4 of 9. Our vet prescribed 5mg Tapazol twice a day, and you verified that that was a high dose and many cats can be controlled with a lower dosage. Based on your answer, I have been giving my cat 2.5mg twice a day. We had her blood work done yesterday, and the t4 count dropped to 1.0!! I am so glad that I gave her only half the amount prescribed by the vet! The vet said that her kidney reading is elevated - I don't know the exact test, but her kidney level went from 30 to 37. 36 was the upper limit of 'normal' for this measurement. The vet wants to recheck the blood again in early September. how do you feel about the kidney count going up?
thank you so much!! ( and what would have happened if I had given her the 5mg twice a day?)

Dr Peterson said.... : Sorry, I deleted your post by mistake. In my lab, a T4 value of 1 ug/dl is low-normal. I like them a bit higher. You might want to lower the dose further to once a day and see if the T4 remains normal (recheck in 2 more weeks). Lowering the T4 too low will cause the kidney values to worsen,

Chandra's mom said...

Thank you Dr Peterson
Is there any benefit of giving 2.5mg once a day rather than 1.25mg twice a day of tapazole? My cat eats the tapazole in her food, so giving it to her twice in a day is no problem (thank goodness!)
Is the elevated kidney readings a result of 'damage' from the tapazole, or does it signal the unmasking of prior kidney problems that were 'hidden' by the hyperthyoidism?

Dr. Mark E. Peterson said...

Twice a day is always better. Hypothyroidism can make renal disease worse. In your cat, it's likely that underlying kidney disease is already present (but it's likely very mild).

Chandra's mom said...

Hello Dr Peterson. Another big big thank you! I have the cat that had a t4 of 9, and my vet prescribed 5mg tapazol twice a day. I wrote to you, and you said that yes, that was a high dose, and that gave me the courage to give her only 2.5mg twice a day. That dropped the t4 to 1.0 (!) and raised the BUN level to 37. My vet said to continue on for another few weeks, but you suggested to cut the dose in half immediately to protect the kidneys. I followed your advice, and have been giving her 1.25mg twice a day. Her test results came in this morning, and Im happy to see a t4 of 3.0, and the BUN at 33. Yes!!!!

But I'm worried about the platelets. She has always had a low count, around 150- 175 in all her tests as she was growing up ( she is turning 15 next week) Years ago, she had blood drawn while she had a urinary infection, and the platelet count was 15!!! yes, just 15. Since she had no other signs of low platelets, we decided to wait a week until the infection cleared and retest. Sure enough, it was back up to around 150. My vet never figured out what caused a count of 15.
Her platelet readings along with the t4 are as follows:

6/26/15 t4 9.6, platelets 147
7/22/15 9.0 189
8/11/15 1.0 175
8/30/15 3.0 107

Your comments on the drop to 107!! have you ever seen a temporary dip in the count, or is this definitely a sign of a real problem developing?
When should I bring her in for another blood sample?

thank you so much!

Dr. Mark E. Peterson said...

Probably nothing to worry about. If the blood clots a bit in the collection tube, you will get a falsely low reading. With methimazole, the platelets don't go up spontaneously and the cats will bleed.