These drugs block the production of thyroid hormones by the thyroid gland. These medications need to be given daily. Hyperthyroidism returns soon after the drugs are stopped. Although methimazole can be used successfully in many cats, it does require careful monitoring, especially during the first few months of treatment. There are a couple of other anti-thyroid drugs available that can be tried for cats that are intolerant of methimazole.
Radioactive iodine is the preferred treatment for many cats.
Because iodine is not used by other body tissues and is only taken up by the thyroid gland. The radioactivity is concentrated in the overactive thyroid gland cells and kills them. The radioactive iodine is then excreted in the urine. Radioiodine is curative for hyperthyroidism in nearly all cases. There are no significant side effects from the treatment itself.