Transient hypothyroidism, with the development of subnormal thyroid hormone levels, is common after radioiodine therapy. However, such suppression in the circulating thyroid hormone concentrations is usually mild and generally does not require thyroid hormone replacement therapy.
Why Transient Hypothyroidism Develops after I-131 Treatment
Prior to treatment of a hyperthyroid cat, the high circulating levels of the thyroid hormones (T3 and T4) have a negative feedback effect on the pituitary gland to suppress thyroid stimulating hormone (TSH) release (1-4). Therefore, serum TSH concentrations in hyperthyroid cats fall to low or undetectable levels (Figure 1).
See the thyroid scans below (Figure 2), which illustrate how cats with a thyroid adenoma affecting only 1 of the 2 thyroid lobes will show complete suppression of the normal lobe and not take up any radioactivity (labeled Unilateral, left panel). In contrast, the cats in the middle and right panels have both thyroid lobes involved with tumor (5).
If not overdosed with radioiodine, almost all these cats have some remaining “normal” thyroid tissue surrounding the thyroid tumor tissue, which has been chronically suppressed by the hyperthyroid state. However, now that the hyperthyroidism has been cured, pituitary TSH secretion eventually recovers and serum TSH concentrations will rise, resulting in stimulation of any remaining normal, but previously dormant, thyroid tissue.
Since transient hypothyroidism is common and and most cats will recover normal thyroid function with time, L-T4 replacement is not generally indicated at the time of this early recheck period.
However, if evidence of new or worsening kidney disease is found, short-term or permanent L-T4 replacement may be indicated to help preserve any residual renal function and prevent irreversible renal injury. I'll be discussing more about hypothyroidism and kidney function in a upcoming post.
- Baral R, Peterson ME. Thyroid gland disorders In: Little SE, ed. The Cat: Clinical Medicine and Management. Philadelphia: Elsevier Saunders, 2012;571-592.
- Mooney CT, Peterson ME. Feline hyperthyroidism In: Mooney CT, Peterson ME, eds. Manual of Canine and Feline Endocrinology Fourth ed. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;199-203.
- Peterson ME. Hyperthyroidism in cats In: Rand JS, Behrend E, Gunn-Moore D, et al., eds. Clinical Endocrinology of Companion Animals. Ames, Iowa Wiley-Blackwell, 2013;295-310.
- Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
- Broome MR. Thyroid scintigraphy in hyperthyroidism. Clin Tech Small Anim Pract 2006;21:10-16.