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
Two alternative treatment methods are ethanol and heat ablation of thyroid nodules. These methods will probably have limited
use in treating hyperthyroidism given the success of medical and radioiodine therapy.
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.