• Clinical science

Antithyroid drugs


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.


Active substance Mechanism of action Onset Indications
  • Rapid onset of action (within a week)


Biosynthesis of thyroid hormones

Side effects



  • Side-effects are rare and often mild.
  • Oral/gastric mucosal irritation (e.g., aphthous ulcers, metallic taste): To avoid mucosal irritation, iodides should be consumed with food or diluted with fluids.
  • Allergy/hypersensitivity: rash, angioedema; rarely, severe anaphylactic reactions can occur
  • Teratogenicity: contraindicated in pregnancy because they can cross the placental barrier and cause fetal goiter
  • Iodides delay and decrease the effects of radioactive iodine. They must be stopped at least a week before radioactive iodine ablation.

Complete blood counts should be monitored in patients taking antithyroid drugs because of the risk of agranulocytosis/aplastic anemia. Liver function tests should be monitored in patients taking propylthiouracil because of the risk of hepatotoxicity.


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