Cats with thyroid carcinoma are also candidates for radioiodine therapy, especially after surgical debulking. In one study, seven cats that had undergone surgical removal of thyroid carcinomas were treated with 30 mCi of radioactive iodine (¹³¹I).⁸ Six cats had clinical signs of hyperthyroidism. The ¹³¹I treatment caused no complications, and the cats' clinical signs resolved. Isotope uptake was not evident on four cats' technetium scans after treatment. In three cats, the isotope uptake was the same or less than the intensity of salivary gland uptake. Hypothyroidism occurred in all cats after treatment; four cats needed L-thyroxine supplementation. One cat was still alive 33 months after treatment. The other six cats were euthanized 10 to 41 months after treatment as a result of unrelated diseases.

Most doses of radioiodine are determined from experience, a combination of clinical and hormonal data (as above), or radioiodine uptake and thyroid volume in a given patient. A recent study evaluated the practicality and accuracy of using thyroid volume estimates in hyperthyroid cats to calculate the dose of radioiodine. For this study, hyperthyroidism was diagnosed in 80 cats with thyroid scintigraphy using technetium pertechnetate.⁹ Based on the volume of hyperfunctioning thyroid tissue indicated in the pertechnetate scans, a dose of radioiodine was calculated and administered to the cats. Several parameters were evaluated in an attempt to identify a difference between treatment success and failure. Cats that failed to become euthyroid after one radioiodine dose had significantly higher pretreatment serum T₄ concentrations, had significantly larger volumes of hyperfunctioning thyroid tissue on scintigrams, and were more likely to have received oral radioiodine therapy. Because of these results, the authors concluded that administering a dose of radioiodine based solely on the volume of hyperfunctioning thyroid tissue estimated from a pertechnetate scan may be inadequate for those patients with extremely elevated serum T₄ concentrations or large thyroid glands. The authors also concluded that oral radioiodine administration should not be recommended for feline hyperthyroidism.

Novel treatment options

Ethanol ablation

Ethanol ablation of thyroid nodules has been tried as a novel nonmedical treatment for hyperthyroidism in cats. This procedure is done under ultrasound guidance with the animal heavily sedated. In one case report, an 8-year-old female spayed domestic shorthaired cat was treated with intrathyroid injections of ethanol.¹⁰ The cat had been only partially responsive to ¹³¹I treatment four weeks before. The percutaneous ethanol injections were given four times over six weeks. The cat became acutely dyspneic shortly after the last treatment. Bilateral laryngeal paralysis was diagnosed and surgically corrected. Repeated measurements of serum T₄ concentrations and thyroid scintigraphy showed a marked decrease in the size and function of the treated thyroid lobe.