A tablet for an overactive thyroid (hyperthyroidism), used for Graves' disease and toxic multinodular goiter or to calm symptoms before thyroid surgery or radioactive iodine. It blocks the thyroid from making new hormone, and its most serious risk is a sudden loss of infection-fighting white blood cells.
Labeled use. Graves' disease or toxic multinodular goiter when surgery or radioactive iodine is not appropriate, and symptom control before thyroidectomy or radioactive iodine.
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Methimazole is usually the first-choice antithyroid drug for Graves' disease and toxic nodular goiter when drug therapy is chosen, because its liver risk is lower than propylthiouracil's, and it is preferred in children. Propylthiouracil is often favored in the first trimester of pregnancy and in thyroid storm, with a switch to methimazole after the first trimester. A beta-blocker is commonly added to control tremor and fast heart rate while thyroid levels fall. Radioactive iodine or thyroidectomy are the definitive alternatives.
An antithyroid drug that inhibits thyroid hormone synthesis. It differs from the thiouracils mainly by having a 5-membered rather than a 6-membered ring. It does not inactivate T4 and T3 already stored in the thyroid or circulating, and it does not block exogenous thyroid hormone. It is well absorbed from the GI tract, then metabolized by the liver and excreted in the urine.
The label does not describe a narrow therapeutic index or direct dosing by measured blood levels of the drug.
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Doses come from the FDA label. Study tool, not for clinical decisions.
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No hepatic dose adjustment on the label, though it is metabolized in the liver. It can cause hepatotoxicity, including acute liver failure: check bilirubin, alkaline phosphatase, ALT, and AST for anorexia, itching, or right upper quadrant pain, and stop promptly for clinically significant abnormality such as transaminases above 3 times the upper limit of normal.
No antidote. Overdose may cause nausea, vomiting, epigastric distress, headache, fever, joint pain, itching, and edema, and aplastic anemia or agranulocytosis can appear within hours to days. Hepatitis, nephrotic syndrome, exfoliative dermatitis, neuropathy, and CNS stimulation or depression are less frequent. Give supportive care, consider other drugs taken, and contact a poison control center.
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It crosses the placenta. First-trimester use has been linked to rare birth defects, including aplasia cutis, facial defects and choanal atresia, esophageal atresia, and omphalocele, so another antithyroid drug may be preferred then; switching from propylthiouracil to methimazole may be preferable in the second and third trimesters. It can cause fetal goiter and cretinism, so give the lowest dose that controls the mother's disease. Untreated Graves' disease itself raises the risk of maternal heart failure, miscarriage, preterm birth, stillbirth, and fetal or newborn hyperthyroidism.
Methimazole is present in breast milk, but several studies, including a long-term study of 139 mothers with hyperthyroidism, found no effect or toxicity in their nursing infants. The label advises thyroid function checks at frequent (weekly or biweekly) intervals.
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Methimazole is the preferred antithyroid drug in children, because propylthiouracil has caused severe liver injury in pediatric patients. Start 0.4 mg/kg per day, split into 3 doses about 8 hours apart; the maintenance dose is about half the starting dose.