Transient Hypothyroidism in Cats Following Radioiodine

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).

Fig. 1: In normal cat, normal levels of thyroid hormone (T4 and T3) are controlled by TSH, a hormone secreted by the pituitary gland. In hyperthyroid cats, their thyroid tumor(s) secrete too much T4 and T3, resulting in a decrease in pituitary TSH secretion. Without adequate amounts of circulating TSH, any remaining normal thyroid tissue stops working and will atrophy.

Since adequate amounts of circulating TSH are needed by normal thyroid tissue for both iodine uptake and thyroid hormone secretion, the decreased circulating TSH concentrations that accompany persistent hyperthyroidism lead to atrophy of the normal (nonadenomatous) thyroid tissue (Figures 1 & 2). In contrast, because the adenomatous thyroid nodules (thyroid tumors) are autonomous and do not require circulating TSH for iodine uptake or thyroid hormone secretion, these thyroid tumors continue to exhibit both growth and hyperfunction (Figure 2).

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).

Fig. 2: Thyroid scans in 3 hyperthyroid cats. All had low serum TSH concentrations. In the cat with a unilateral thyroid adenoma (left panel), the normal thyroid lobe is not visible — it has stopped functioning properly and has atrophied as a result of the low circulating TSH concentrations.

Many cats treated with radioiodine go through a period of transient, mild hypothyroidism after the 131-I treatment has destroyed their thyroid tumor(s). When rechecked within the first month after radioiodine therapy, many hyperthyroid cats have low to low-normal serum T4 concentrations (Figure 3).

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.

Fig. 3: In hyperthyroidism,  the high T4 and T3 levels suppress pituitary TSH secretion to very low levels. Once the thyroid tumor is irradiated and destroyed, the T4 and T3 levels will fall to normal or even low levels for 1-2 months. With time, however, the pituitary gland recovers and starts to secrete TSH once again (sometimes to higher-than-normal amounts for a time). This allows any remaining normal thyroid tissue to become active once again and secrete normal amounts of thyroid hormone.

Most cats with a sufficient volume of normal thyroid tissue regain the ability to produce adequate amounts of thyroid hormone to maintain euthyroidism within 3 months of radioiodine therapy. Rarely, cats require as long as 6 months of chronically increased endogenous TSH levels to recover lost thyroid function and regain the ability to maintain normal thyroid hormone levels.

Bottom Line
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.

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. 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.
3. 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.
4. 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.
5. Broome MR. Thyroid scintigraphy in hyperthyroidism. Clin Tech Small Anim Pract 2006;21:10-16. 

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