Do I need to stop the antithyroid medications (Tapazole or PTU) prior to 131I treatment?
Yes. They should be discontinued approximately 1 week before treatment. Exceptions can be made if your cat has serious cardiac disease. Cats with significant cardiac disease, which cannot be easily controlled even with cardiac drugs, may need to remain on Tapazole until 48 hours prior to 131I . If this is a factor, you will be instructed by your veterinarian or by me. If your cat can tolerate being without these medications for longer, then discontinue them even sooner. The antithyroid medications may decrease 131I uptake by the abnormal thyroid tissue, and therefore will decrease its effectiveness. So, the longer they can be off these medications without compromising their health the better–but 1 week seems to be fine.
