Methimazole is used to treat hyperthyroidism in cats. Methimazole is used to treat overactive thyroid (hyperthyroidism). It is also used before thyroid surgery or radioactive iodine treatment. It has largely replaced propylthiouracil in this treatment process since it has a lower incidence of adverse side effects. Methimazole requires a prescription from your veterinarian, and is sold per tablet.