Boshoven E.W. and Conway T.S.
American College of Veterinary Radiologists, 2012. p.96.
Introduction/Purpose: Radioactive Iodine-131 (131I) has long been used for the treatment of patients with hyperthyroidism. Little information is available pertaining to the outcome of such treatments in cats. In an effort to expand this knowledge, bloodwork was obtained at three weeks and three months following 131I treatment of hyperthyroidism in 90 cats.
Methods: To date, 90 cats have been injected with 131I for the treatment of hyperthyroidism at our facility. The dose of 131I was determined based on a combination of the peak total T4 (TT4), the response to methimazole (if given) and blood urea nitrogen (BUN), serum creatinine (Cr) and urine specific gravity levels. Nuclear imaging was not available for any patient. Bloodwork was performed at approximately three weeks and three months following 131I injection by our hospital or the referring veterinarian. 131I was provided by a commercial nuclear pharmacy and arrived pre- calibrated.
Results: To date, 73 patients treated with 131I were rechecked at three weeks post 131I injection, 56 patients were rechecked three months post 131I injection and 48 patients have been evaluated at both three weeks and three months post 131I injection. At three weeks, 37 patients (51%) were hypothyroid (TT4 < 1.0μg/dl), 30 (41%) were euthyroid (TT4 1.0μg/dl – 4.0μg/dl) and six (8%) were hyperthyroid (TT4 > 4.0μg/dl). At three months, 20 of 56 were hypothyroid (36%), 33 were euthyroid (59%) and three were hyperthyroid (5%). Of the 20 hypothyroid cats at the three month point, fourteen (70%) were hypothyroid at the three week point, one was hyperthyroid (5%) and five (26%) were euthyroid. Of the three cats that were hyperthyroid at the three month point, one (33%) was hyperthyroid at three weeks, one (33%) was euthyroid and one (33%) did not have blood drawn at three weeks.
The average pre-treatment TT4 for cats that became hypothyroid at three months following treatment was 9.6μg/dl (median 9.1, range 2.5-20). These cats received an average dose of 4.1mCi (median 4.0, range 3.5-5.5) of 131I. They were diagnosed as hyperthyroid for an average of 108 days prior to treatment (median 52, range 5-780). The average pre-treatment TT4 for cats that were still hyperthyroid at three months following 131I treatment was 11.3μg/dl (median 13.4, range 6.9-13.7). They received an average dose of 4.5mCi (median 4.5, range 4-5). These cats were diagnosed as hyperthyroid for an average of 428 days (median 367, range 17-901) prior to treatment with 131I.
Discussion/Conclusion: Follow-up after 131I injection is important to determine the efficacy and possible adverse outcomes such as iatrogenic hypothyroidism or persistent hyperthyroidism. Additional means of determining the ideal 131I dose may improve our ability to achieve euthyroidism and avoid iatrogenic hypothyroidism or persistent hyperthyroidism.