RADIOIODINE TREATMENT OF 524 CATS WITH HYPERTHYROIDISM

Objective-To evaluate a protocol for subcutaneous radioiodine treatmen t of cats with hyperthyroidism in which the dose was determined on the basis of severity of the cat's clinical signs, thyroid tumor size, an d magnitude of the serum thyroxine (T-4) concentration. Design-Prospec tive case series. Animals-524 cats with hyperthyroidism. Procedure-A s coring system based on 3 factors (severity of clinical signs, size of the thyroid gland, and magnitude of the serum T-4 concentration) was u sed to select the dose of radioiodine to be administered subcutaneousl y. Results-On the basis of the scoring system, 310 (59%) cats were tre ated with a low dose of radioiodine (< 3.5 mCi; median, 3.0 mCi), 158 (30%) were treated with a moderate dose (3.5 to 4.4 mCi; median, 4.0 m Ci), and 56 (11%) were treated with a high dose (greater than or equal to 4.5 mCi; median, 5.0 mCi). At lime of discharge from the hospital, serum T-4 concentration was still high in 80 (15.3%) cats, but by 6 m onths after administration of radioiodine, the serum T-4 concentration had decreased to within or below reference range in all but 8 (1.5%) cats with persistent hyperthyroidism. Many cats had low serum T-4 conc entrations at some time after radioiodine treatment, but only 11 (2.1% ) cats developed clinical and clinicopathologic features of hypothyroi dism and required supplementation with L-thyroxine. Thirteen (2.5%) ca ts had a relapse of hyperthyroidism 1.1 to 6.5 years after initial rad ioiodine treatment. Overall, the response to treatment was considered good in 94.2% of the cats. Median survival lime in the cats was 2.0 ye ars; the percentage of cats alive after 1, 2, and 3 years of treatment was 89, 72, and 52%, respectively. Clinical Implications-Results of t he study suggest that this method of dose estimation works well and th at subcutaneous administration of radioiodine provides a safe and effe ctive means of treating hyperthyroidism in cats.