Outcome of Radioactive Iodine Therapy in Cats Receiving Recent Methimazole Therapy

Oman R. and Lunn K.F.

Conference Proceedings, (2011). American College of Veterinary Internal Medicine:


Radioactive iodine (131I) is a widely used treatment for feline hyperthyroidism. Prior to 131I administration, many cats receive methimazole therapy. It has been suggested that recent withdrawal of methimazole prior to 131I may increase the risk of hypothyroid- ism, inhibit the response to therapy, or have no effect. To further address this question, a retrospective medical records search was performed to identify hyperthyroid cats that received 131I therapy after methimazole treatment. Inclusion criteria included documenta- tion of the time interval between discontinuation of methimazole and 131I administration, and measurement of thyroxine (T4) at 7–14 days after 131I. Cats were divided into 2 groups: those receiving 131I within 1 day of stopping methimazole, and those receiving 131I treatment 5 or more days after stopping methimazole.

Sixty cats met the inclusion criteria. Forty received 131I within 1 day of stopping methimazole. Of those, 20 (50%) had a low T4 (o 1.2 mcg/dl), 17 (42.5%) had a normal T4 (1.2–4.8 mcg/dl), and 3 (7.5%) had an elevated T4 (4 4.8 mcg/dl) at 7–14 days after 131I therapy. Fourteen cats received 131I 5 or more days after stopping methimazole: 8 (57%) had a low T4, 5 (36%) had a normal T4, and 1 (7%) had an elevated T4 at 7–14 days after 131I therapy. The results were compared with a Fisher’s exact test and there was no difference between the groups (p 5 0.76). These findings indicate that stopping methimazole therapy within 1 day of 131I therapy does not inhibit the response to therapy.