Antithyroid drug: A drug directed against the thyroid gland. The antithyroid drugs include carbimazole, methimazole, and propylthiouracil (PTU). These drugs are used to treat hyperthyroidism (overactivity of the thyroid gland) in order to reduce the excessive thyroid activity before surgery and to treat and maintain patients not having surgery.

Carbimazole, its active metabolite methimazole, and propylthiouracil all act by inhibiting the enzyme thyroid peroxidase and in that way they block the synthesis (the production) of thyroid hormone.

About 30 to 40% of patients treated with an antithyroid drug remain euthyroid (with normal levels of thyroid hormone) 10 years after the discontinuation of antithyroid drug therapy, which means that the Graves' disease (the most common cause of hyperthyroidism) is in remission.

A common problem with antithyroid drugs is undershooting or overshooting causing persistent hyperthyroidism or hypothyroidism. A rare complication of antithyroid therapy is agranulocytosis (decrease in white blood cells) which calls for immediate discontinuation of the drug.