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Antithyroid drugs

Last updated: May 28, 2021

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Antithyroid drugs are drugs that either decrease thyroid hormone synthesis (thionamides) or thyroid hormone release (iodides). The most important group of antithyroid drugs are thionamides, which include methimazole, carbimazole, and propylthiouracil. Thionamides decrease thyroid hormone synthesis by inhibiting thyroid peroxidase, an essential enzyme involved in multiple steps of thyroid hormone synthesis. Thionamides play a significant role in the treatment of hyperthyroidism and thyroid storm. Iodides are solutions containing potassium iodide that inhibit the release of thyroid hormones into the circulation. They are, therefore, a useful adjunct treatment in the management of thyroid storm, but their main clinical use is for the preoperative preparation of a patient with Graves' disease, because they decrease the vascularity of the thyroid gland.

Overview of antithyroid drugs
Thionamides Potassium iodides
  • Methimazole
  • Carbimazole
  • Propylthiouracil
Mechanism of action
  • Rapid onset of action (within a week)
Special considerations


See thyroid hormone synthesis.


As methimazole and carbimazole are teratogenic, propylthiouracil is recommended in the first trimester. After the first trimester, switch back to carbimazole or methimazole because of the hepatotoxic effects of propylthiouracil.



We list the most important adverse effects. The selection is not exhaustive.

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