antithyroid drug

An antithyroid drug turns down a thyroid gland that is running too hot. In hyperthyroidism the gland floods the body with excess thyroid hormone, speeding the heart, melting weight away, and fraying the nerves; these medicines work by jamming the factory line so the gland makes less hormone, gradually bringing an overactive metabolism back toward normal.

The main agents, called thionamides — methimazole, its prodrug carbimazole, and propylthiouracil — block thyroid peroxidase, the enzyme that stitches iodine onto thyroglobulin to build thyroid hormone. Because they stop new synthesis but do not destroy hormone already stored in the gland, their full effect takes several weeks to appear as the existing reserve is used up.

Antithyroid drugs are a mainstay for Graves' disease, often given for a one-to-two-year course in hopes of remission, and to prepare patients for surgery or radioiodine. They carry uncommon but serious risks, most notably agranulocytosis — a sudden dangerous drop in infection-fighting white cells — and liver injury, so patients are warned to report fever or sore throat at once. Propylthiouracil has an extra role in the first trimester of pregnancy, where it is preferred over methimazole.