What is methimazole used for?

What Methimazole Is Used For

Methimazole is an antithyroid medication used to treat hyperthyroidism, specifically in patients with Graves' disease or toxic multinodular goiter, and to prepare patients for definitive therapy with surgery or radioactive iodine. 1

Primary Indications

• Graves' disease with hyperthyroidism in patients for whom surgery or radioactive iodine therapy is not an appropriate treatment option 1
• Toxic multinodular goiter when definitive treatments are not suitable 1
• Preparation for thyroidectomy or radioactive iodine therapy to ameliorate hyperthyroid symptoms before definitive treatment 1

Mechanism and Clinical Use

Methimazole works by inhibiting thyroid hormone synthesis within the thyroid gland at multiple steps 2, 3. The typical starting dose ranges from 10-30 mg as a single daily dose, with the advantage of once-daily dosing compared to propylthiouracil which requires dosing every 6 hours 2, 3.

Methimazole is the preferred antithyroid drug over propylthiouracil in most clinical situations because it has less frequent major side effects, allows single daily dosing, is less expensive, and is more widely available 2, 3. The standard treatment duration is typically 12-18 months before considering discontinuation 4.

Important Safety Considerations and Contraindications

Pregnancy-Specific Guidance

• Methimazole is contraindicated in the first trimester of pregnancy due to teratogenic effects, including aplasia cutis congenita, choanal atresia, and esophageal atresia 5, 2
• Propylthiouracil is the drug of choice during the first trimester 5
• Switch to methimazole in the second and third trimesters due to propylthiouracil's hepatotoxicity risk 5
• Women of childbearing age require preconception counseling, as methimazole normalizes ovulatory function and increases unplanned pregnancy risk 5

Adverse Effects Profile

• Serious reactions occur in less than 5% of cases, including agranulocytosis (3 per 10,000 patients), hepatitis, vasculitis, and thrombocytopenia 6, 5
• Common minor side effects include rash and pruritus 7
• Patients must be educated about agranulocytosis symptoms (sore throat, fever) requiring immediate medication discontinuation 6

Monitoring Requirements

• Monitor thyroid function (TSH and FT4) every 4-6 weeks during initial treatment and dose adjustment 6
• After discontinuation, monitor every 1-3 months for the first year to detect recurrent hyperthyroidism 4
• Regular liver function monitoring is advisable, though hepatotoxicity is less common with methimazole than propylthiouracil 6

Alternative Formulations and Dosing Strategies

For patients with moderate to severe hyperthyroidism (free T4 ≥5 ng/dL), combining methimazole 15 mg with inorganic iodine 38 mg daily achieves faster control with fewer adverse effects compared to methimazole 30 mg daily alone 8. This combination approach resulted in significantly more patients achieving euthyroid status within 30-60 days (45.3% vs 24.8% at 30 days) and fewer adverse effects requiring drug discontinuation (7.5% vs 14.8%) 8.

Methimazole ointment (5% applied to skin around the thyroid) has demonstrated similar clinical efficacy to oral tablets but with significantly fewer systemic adverse effects (1.5% vs 12.3%) 9.

When Definitive Treatment Should Be Considered

Radioactive iodine therapy or thyroidectomy should be considered for patients who cannot tolerate thionamide therapy, do not respond adequately, or experience recurrent hyperthyroidism after medication discontinuation 6, 4. Radioactive iodine is contraindicated in pregnancy 6.

Common Pitfalls to Avoid

• Do not use methimazole in the first trimester of pregnancy—switch to propylthiouracil 5
• Do not continue methimazole if agranulocytosis or severe hepatotoxicity develops—these are absolute contraindications to desensitization 7
• Do not assume all patients can safely discontinue after 12-18 months—close monitoring for recurrence is essential, as relapse rates are significant 4
• For patients requiring G-tube administration, methimazole cannot be crushed; propylthiouracil is the alternative that can be crushed and administered via G-tube with a typical conversion ratio of 10:1 6