Methimazole

What it is and how it works

Mechanism class: Thyroid Hormone Synthesis Inhibitors

Indications

INDICATIONS AND USAGE Methimazole tablets are indicated: In patients with Graves’ disease with hyperthyroidism or toxic multinodular goiter for whom surgery or radioactive iodine therapy is not an appropriate treatment option. To ameliorate symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy.

Dosage and administration (summary)

DOSAGE AND ADMINISTRATION Methimazole tablets are administered orally. The total daily dosage is usually given in 3 divided doses at approximately 8-hour intervals. Adult - The initial daily dosage is 15 mg for mild hyperthyroidism, 30 to 40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. The maintenance dosage is 5 to 15 mg daily. Pediatric - Initially, the daily dosage is 0.4 mg/kg of body weight divided into 3 doses and given at 8-hour intervals. The maintenance dosage is approximately ½ of the initial dose.

Contraindications

CONTRAINDICATIONS Methimazole tablets are contraindicated in the presence of hypersensitivity to the drug or any of the other product components.

Warnings and precautions

WARNINGS First Trimester Use of Methimazole and Congenital Malformations Methimazole crosses the placental membranes and can cause fetal harm, when administered in the first trimester of pregnancy. Rare instances of congenital defects, including aplasia cutis, craniofacial malformations (facial dysmorphism; choanal atresia), gastrointestinal malformations (esophageal atresia with or without tracheoesophageal fistula), omphalocele and abnormalities of the omphalomesenteric duct have occurred in infants born to mothers who received methimazole tablets in the first trimester of pregnancy.

Pregnancy

Pregnancy If methimazole tablets are used during the first trimester of pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus.

Storage and supply

Store at 20° to 25°C (68° to 77°F) . Dispense in a tight, light-resistant container.

Source: Methimazole (Methimazole) — US FDA prescribing information, 2026-08-20, DailyMed. Summarised and shortened; the full label governs. Iranian registration, indications and labelling may differ.