Methimazole Dosing for Hyperthyroidism
For adults with hyperthyroidism, initiate methimazole at 15 mg daily for mild disease, 30-40 mg daily for moderate disease, or 60 mg daily for severe disease, with maintenance doses of 5-15 mg daily once euthyroid. 1
Initial Dosing Strategy
The FDA-approved dosing regimen provides clear severity-based guidance 1:
- Mild hyperthyroidism: 15 mg daily
- Moderately severe hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily
Traditionally, the FDA label recommends dividing the total daily dose into 3 doses given at 8-hour intervals 1. However, single daily dosing is equally effective and may reduce side effects 2, 3. Research demonstrates that 15 mg as a single daily dose achieves euthyroidism in 93% of patients within 12 weeks, with a mean time of 5.3 weeks, and causes fewer adverse effects than divided dosing 2. Long-term follow-up confirms no difference in remission rates between single daily dosing (15 mg once daily) versus conventional divided dosing (10 mg three times daily), but adverse effects occurred less frequently with single daily dosing (13% vs 24%) 3.
Maintenance and Monitoring
Reduce to maintenance doses of 5-15 mg daily once the patient becomes euthyroid 1. The American College of Clinical Endocrinologists recommends maintaining free T4 in the high-normal range using the lowest effective dose, with monitoring of free T4 or Free T4 Index every 2-4 weeks during initial treatment 4.
If TSH becomes suppressed during therapy, this signals overtreatment or recovery of thyroid function—reduce or discontinue the dose with close follow-up 4.
Adjunctive Therapy
Add a beta-blocker (propranolol or atenolol) for symptomatic relief until thyroid hormone levels normalize 4. This is particularly important in the initial weeks of treatment when patients remain symptomatic despite starting antithyroid medication.
Critical Safety Considerations
Agranulocytosis is a serious adverse effect that typically presents with sore throat and fever 4. If these symptoms develop:
- Obtain immediate complete blood count
- Discontinue methimazole immediately 4
The risk of hematopoietic damage is dose-dependent, making lower maintenance doses preferable 5.
Special Population: Pregnancy
In pregnancy, propylthiouracil is preferred over methimazole during the first trimester due to lower risk of birth defects 4. Methimazole has been associated with aplasia cutis and choanal/esophageal atresia 6. After the first trimester, switching to methimazole may be considered given its more favorable side effect profile in non-pregnant patients.
Pediatric Dosing
For children, the initial daily dosage is 0.4 mg/kg of body weight divided into 3 doses at 8-hour intervals, with maintenance dosing at approximately half the initial dose 1.