antithyroid drug

An antithyroid drug turns down a thyroid gland that is running too hot. In hyperthyroidism the gland makes too much thyroid hormone, speeding up the whole body, racing heart, weight loss, anxiety, heat intolerance. These drugs slow the factory by interfering with how the gland builds its hormone, gradually bringing levels back to normal.

The main agents are thionamides: carbimazole (and its active form methimazole) and propylthiouracil. They inhibit thyroid peroxidase, the enzyme that attaches iodine to tyrosine residues to build thyroid hormone, so the gland's new production falls. Because they block synthesis rather than destroy stored hormone, the effect takes weeks to appear as existing hormone is used up. Propylthiouracil has an extra action of blocking the peripheral conversion of T4 to the more active T3, useful in severe cases.

These drugs are used to control Graves' disease and other causes of hyperthyroidism, either as definitive treatment over many months or to prepare a patient before radioactive iodine or surgery. The most serious adverse effect is agranulocytosis, a sudden severe drop in infection-fighting white cells, so patients are told to report fever or sore throat urgently and stop the drug. Propylthiouracil carries a risk of severe liver injury but is preferred in early pregnancy, where methimazole is more strongly linked to birth defects.