Methimazole Dosing for Hyperthyroidism in Adults
For adults with hyperthyroidism, start methimazole at 15 mg daily for mild disease, 30-40 mg daily for moderate disease, and 60 mg daily for severe hyperthyroidism, divided into three doses at 8-hour intervals. 1
Initial Dosing Strategy
The FDA-approved dosing regimen stratifies initial therapy based on disease severity 1:
- Mild hyperthyroidism: 15 mg daily
- Moderately severe hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily
All doses should be divided into three administrations at approximately 8-hour intervals 1.
Important Dosing Considerations
The starting dose should not exceed 15-20 mg/day when possible, as adverse effects—particularly agranulocytosis—are dose-dependent. 2 This creates a clinical tension between the FDA labeling and safety data, particularly for moderate to severe disease.
Single Daily Dosing Option
Research demonstrates that single daily dosing of 15 mg methimazole is effective for most patients with Graves' hyperthyroidism and causes fewer adverse effects than higher divided doses 3, 4:
- 93% of patients achieved euthyroid status within 12 weeks on 15 mg single daily dose 3
- Mean time to euthyroidism was 5.3-5.8 weeks, comparable to divided dosing regimens 3, 4
- Adverse effects occurred in only 13% with single daily dosing versus 24% with conventional divided doses 4
Enhanced Regimen for Moderate-Severe Disease
For patients with moderate to severe hyperthyroidism (free T4 ≥5 ng/dL), combining methimazole 15 mg daily with inorganic iodine 38 mg daily achieves faster control than methimazole 30 mg alone, with fewer adverse effects 5:
- 45% achieved normal free T4 within 30 days (versus 25% with methimazole 30 mg alone) 5
- Adverse effects requiring discontinuation: 7.5% versus 14.8% with higher methimazole dose 5
- Discontinue iodine once free T4 normalizes 5
Maintenance Dosing
Once euthyroid status is achieved, reduce to a maintenance dose of 5-15 mg daily 1. Titrate based on thyroid function tests, typically checked every 4-6 weeks initially 6.
Critical Safety Warnings
- Propylthiouracil should NOT be used as first-line therapy due to risk of severe liver failure requiring transplantation or causing death 2
- Propylthiouracil is reserved only for first trimester pregnancy or methimazole intolerance 2
- Monitor for agranulocytosis: if sore throat or fever develops, obtain complete blood count immediately and discontinue methimazole 6
- Other adverse effects include hepatitis, vasculitis, and thrombocytopenia 6
Special Populations
In pregnancy, methimazole can be used safely (except first trimester where propylthiouracil is preferred), with the goal of maintaining free T4 in the high-normal range using the lowest possible dose 6. Women can breastfeed safely while taking methimazole 6.