Wednesday, September 11, 2013

Diagnosing Naturally Occurring Hypothyroidism in Cats


I have a 5-year-old female DSH cat that has recently been diagnosed with hypothyroidism.  This diagnosis was based on a low serum T4 value that was detected during a pre-anesthetic workup for a preventative dental procedure. She had been looking unthrifty and "off" to me at home for a while, but has had no other signs of illness. Her appetite remains normal with no signs of vomiting or diarrhea.

We sent out a complete serum thyroid panel two weeks after the initial low T4 value and the results were as follows:
  • Total T4: 0.3 µg/dL (reference range, 0.8-4.7 µg/dL)
  • Free T4 - 0.35 ng/dL (reference range, 0.7-2.6 ng/dL)
  • Total T3: <35 ng/dL (52-182 ng/dL)
  • TSH 0.01 ng/ml (reference range, 0.05-0.42 ng/ml)
How would you interpret this panel?  Should we treat her as being hypothyroid? She has never been hyperthyroid and has never been treated with methimazole or radioiodine.

Is there value in trying to use one of the herbal remedies  for thyroid support (e.g., Thytrophin PMG) or should we just start her on thyroid hormone? If you think we should use thyroid hormone, which product is best to use (L-T4, L-T3, or a combination)?

My Response:

Based on your cat's serum thyroid panel, the results are not supportive of primary hypothyroidism. As the thyroid gland fails in hypothyroidism, the pituitary would respond by increasing the secretion of thyroid stimulating hormone (TSH) (1-4). In your cat, all of the serum thyroid hormone values (T4, T3, and free T4) are very low. However, the serum TSH level remains well within the reference range limits, not high as would be expected with primary hypothyroidism.

Again, the result of this thyroid panel goes against a diagnosis of primary hypothyroidism. Spontaneous hypothyroidism (i.e., not secondary to treatment of hyperthyroidism) is extremely rare in the adult cat, with only a few cases reported (4,5), so I'm not surprised that the serum TSH value is normal in your cat.

The most likely explanation for these serum thyroid results is that your cat is likely suffering from another illness. It's well known that any nonthyroidal disease can act to lower circulating T4 and T3 concentrations in cats (6-10); thyroid hormone replacement in those cats would unlikely be of any benefit.

I'm assuming that you did routine blood work (CBC, serum chemistry panel, urinalysis) in addition to the T4 for the pre-dental panel. Were there any abnormalities on this blood-work or urine?  If not, a chest radiography or abdominal ultrasound may be indicated as the next step to search for the underlying problem. Once we determine what's wrong with your cat and treat the primary problem, the serum thyroid values will likely normalize.

In any case, supplementation with thyroid hormone or any one of the herbal remedies are unlikely to be of any use and have the potential to do harm (11,12), depending on the underlying cause of your cats problems.  If your cat was indeed hypothyroid (which she isn't!), we generally use twice daily L-T4 replacement, but some cats do appear to respond better to combination L-T4/L-T3 therapy.

References:
  1. Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
  2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: Hypothyroidism. Compend Contin Educ Vet 2013;35:E1-E6. 
  3. Greco DS. Diagnosis of congenital and adult-onset hypothyroidism in cats. Clin Tech Small Anim Pract 2006;21:40-44. 
  4. Daminet S. Feline hypothyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;1-5.
  5. Rand JS, Levine J, Best SJ, et al. Spontaneous adult-onset hypothyroidism in a cat. J Vet Intern Med 1993;7:272-276. 
  6. Peterson ME, Melian C, Nichols R. Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. J Am Vet Med Assoc 2001;218:529-536. 
  7. Peterson ME, Gamble DA. Effect of nonthyroidal illness on serum thyroxine concentrations in cats: 494 cases (1988). J Am Vet Med Assoc 1990;197:1203-1208. 
  8. Mooney CT, Little CJ, Macrae AW. Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats. J Am Vet Med Assoc 1996;208:2004-2008. 
  9. McLoughlin MA, DiBartola SP, Birchard SJ, et al. Influence of systemic nonthyroidal illness on serum concentrations of thyroxine in hyperthyroid cats. J Am Anim Hosp Assoc 1993;29:227-234. 
  10. Mooney CT. The effects of nonthyroidal factors on tests of thyroid function. Australian College of Veterinary Scientists, Science Week 2010;28-30.  
  11. Bello G, Paliani G, Annetta MG, et al. Treating nonthyroidal illness syndrome in the critically ill patient: still a matter of controversy.  Curr Drug Targets 2009;10:778-787. 
  12. Adler SM, Wartofsky L. The nonthyroidal illness syndrome. Endocrinol Metab Clin North Am 2007;36:657-672, vi.

6 comments:

DMZ said...

Dr. Peterson: I just got back my 3rd set of labs within a year (2nd within 2 months), and my vet and I are stumped about what is going on with my 11 y.o. DSH. He is showing hyperthyroid symptoms (he looks exactly like that emaciated cat you have pictured on the "10 most common symptoms" post, with the lumbar spinal area abdominal pocket due to weight loss, despite eating like a horse), but the labs show otherwise:

Dropping thyroid and increasing BUN levels. BUN: July 2012: 21. June 2013: 35. Aug 2013: 40. (The creatinine (1.9, 2.1, 2.2) and phosphorus levels (4.4, 3.7, not tested in Aug 2013) are high normal range.)

Thyroid: 3.3, 2.4, 1.8 but my vet says he's not hyperthyroid.

His weight has dropped:
Dec 2012: 12.1 lbs. June 2013: 10 lbs. Aug. 2013: 9 lbs 14 oz.

Can you point me in the right direction as to what labs or next steps I should take? My vet and I are all out of ideas--the best we have come up with is borderline CKD--and I can't go the route of expensive speculation via ultrasound, exploratory surgery etc.

It seems that thyroid issues are hard to detect because the labs show one reality, and the patients physical symptoms indicate the opposite. It's very frustrating.

Thanks for any guidance you can offer.

Dr. Mark E. Peterson said...

Based on the serum creatinine and BUN values, all are a bit too high, diagnostic for stage 2 chronic kidney disease. That could certainly explain some of the weight loss. See this website for more information: http://www.iris-kidney.com/

The total T4 values are in the upper range for normal, however, so your cat may be mildly hyperthyroid too. The renal disease might suppress the T4 value slightly and can make the diagnosis of hyperthyroidism more difficult to make.

Can your vet palpate a thyroid nodule? In any case, I'd do a complete serum thyroid panel, including a total T4 (ideally by RIA), free T4 by dialysis, and TSH level. If the free T4 is high and the TSH undetectable, that would be consistent with mild hyperthyroidism.

Tarah Poochigian said...

Hi Dr. Peterson,

I have a 5 month old kitten just diagnosed with hypothyroidism. He had been constipated for several days even after several enemas. The original diagnosis was megacolon and I was told his prognosis was grave. After consulting another doctor, we checked his t4 and sure enough it was very low (<0.5) and he does have a palatable thyroid nodule. We confirmed with a free T4 EQ that came back as low with 4.4 (range 10-50). He seems to be doing great with the levothyroxine now, but I am concerned about the constipation returning. He currently gets lactulose BID and I had some cisopride compounded for him before we had the results for the t4 tests. Do you think he will be at risk for constipation without the help of medications?

His chemistry and CBC were both unremarkable. I am hoping that he will start to grow a bit now with the levothyroxine therapy. I also have his litter mate who also seems a bit small, but is asymtomatic.

I have been an RVT for 11 years and this is the first hypothyroid kitten I have ever seen. Any advice would be great, thanks!

Dr. Mark E. Peterson said...

First of all, the finding of a low total and free T4 alone is not diagnostic for hypothyroidism, especially in a cat with other problems (megacolon). The goiter is certainly suggestive but we should confirm the diagnosis. To do that, I'd withhold the L-T4 medication for a couple of days and run a TSH value. If the cat is hypothyroidism, the serum TSH concentration should come back very elevated.

Treating cats with congenital hypothyroidism generally helps the constipation but may not resolve the problem. Part of the issue is that hypothyroid cats are not always easy to regulate. We need to give the LT4 twice daily, generally without food to increase the absorption. Sometimes we need to add in LT3 to the treatment regime to get the T4 and T3 normalized and lower the high TSH into the reference range.

Bottom line -- treating these cat is NOTHING like treating the typical dog with hypothyroidism. It's much more difficult to establish euthyroidism.

So to start, it's critical to make sure that your cat(s) really have hypothyroidism. (I'd check the other cat too - measure T4, T3, FT4, and TSH)

Jennifer LaBelle said...

My 14-year old Balinese cat Bianca has a very unusual situation with Hypothyroid values that were suspected as being Euthroid Sick Syndrome due to chronic renal disease. However, further testing via first Free T3/ Free T4 values and then a full thyroid panel have shown values resulting in us not being able to rule out primary hypothyroidism. She has never had hyperthyroidism, and thus no I-131 treatment.

She showed physical symptoms one would associate with hypothyroidism in spring 2012. In June 2012, she was diagnosed with chronic renal disease and her Free T4 was 0.6. Treatments for renal disease was started and repeat labs were run twice and renal associated values improved. The Free T4 then continued to decrease, bottoming out at 0.2. The vet started Bianca on Soloxine 0.1 mg per day, which increased her T4 over time to its current level of 2.0.
Recent full panel run 1/18/14, however is baffling:
Total Thyroxine (TT4): 33
Total Triiodothyronine (TT3): 0.5 L
Free Thyroxine (FT4): 13
Free Triiodothyronine (FT3): 1.6
Thyroid Stimulating Hormone: 70 H (Range 0-21)

Are there other cases where ESS has had this much of a continuing impact on TSH and T3? I am considering asking our vet to introduce T3 supplementation (Cytomel)as well but am trying to determine if this may be true hypothyroidism, in addition to renal disease.

Thank you,
Jennie and Bianca

Dr. Mark E. Peterson said...

Spontaneous hypothyroidism is rare in adult cats, with only 2 cases ever reported. As you stated, the low serum thyroid values all may have been secondary to nonthyroidal disease, so we can't consider those diagnostic for hypothyroidism. The current high serum TSH value (on LT4) could be due to hypothyroidism but could also reflect illness or lab error. Ideally, we would diagnose hypothyroidism based on either a thyroid scan and/or a thyroid biopsy, especially since we know that it is so very uncommon. In other species (dogs, humans), naturally occurring hypothyroidism is a young adult disease, not one that tends to first show up during the senior years.

The dose of L-T4 (0.1 mg) per day will not be enough to treat most cats with iatrogenic hypothyroidism anyway. You could raise the dose of L-T4 to see if the TSH falls (75-100 µg, twice daily), but none of this makes much sense to be honest. Once we start LT4 supplementation, it would take 2-3 months of being off of the drug to really get a good reevaluation of the thyroid status, and you probably don't want to do that.