Antithyroid drugs

Last updated: May 8, 2023

Summarytoggle arrow icon

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.

Overviewtoggle arrow icon

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


Biosynthesis of thyroid hormonestoggle arrow icon

Adverse effectstoggle arrow icon


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.

Referencestoggle arrow icon

  1. Ross DS. Thyroid storm. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated: January 30, 2017. Accessed: February 21, 2017.
  2. Ross DS. Pharmacology and toxicity of thionamides. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated: October 17, 2016. Accessed: February 21, 2017.
  3. Katzung B,Trevor A. Basic and Clinical Pharmacology. McGraw-Hill Education ; 2014
  4. UpToDate. Potassium iodide (SSKI, Thyroshield): Drug information. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. Last updated: January 1, 2017. Accessed: March 22, 2017.
  5. Katzung BG. Basic and Clinical Pharmacology. McGraw-Hill Publishing Co ; 2004
  6. Ganong WF. Review of Medical Physiology. Appleton & Lange ; 2001
  7. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology (Enhanced EB). McGraw Hill Professional ; 2009

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 Evidence-based content, created and peer-reviewed by physicians. Read the disclaimer