Antithyroid drugs are widely used in human medicine for the medical management of Graves’ disease. Because patients with Graves’ disease may undergo spontaneous remission, antithyroid drugs are preferred for long-term therapy because they do not permanently affect thyroid function. Hyperthyroidism in cats is somewhat different, in that spontaneous remission has not been reported and therefore ablative treatment (surgery or radioiodine) is often preferred. However, antithyroid drugs are essential for preoperative stabilization of cats with hyperthyroidism and often are used for long-term management of certain cases. This chapter will review the various drugs available for the medical management of hyperthyroidism, their mechanisms of action, indications for use, and adverse side effects.