Methimazole (Neomarcazole) Dosing for Hyperthyroidism
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 disease, with maintenance doses of 5-15 mg daily once euthyroid. 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
Traditionally, these doses were divided into three administrations at 8-hour intervals, but single daily dosing is equally effective and may reduce adverse effects 2. A single daily dose of 15 mg achieves euthyroidism in 93% of patients within 12 weeks, with mean time to euthyroidism of 5.3 weeks 2. The intrathyroidal concentration of methimazole remains therapeutic for at least 26 hours, supporting once-daily administration 3.
Dose Optimization Considerations
Doses above 15 mg daily do not significantly increase intrathyroidal drug concentrations due to saturable thyroid uptake mechanisms 3. However, for patients with moderate to severe hyperthyroidism (free T4 ≥5 ng/dL), combining methimazole 15 mg with inorganic iodine 38 mg daily achieves faster normalization of thyroid function compared to methimazole 30 mg alone, while causing fewer adverse effects requiring drug discontinuation (7.5% vs 14.8%) 4.
Maintenance and Monitoring
- Target free T4 in the high-normal range using the lowest effective methimazole dose 5
- Monitor free T4 or Free T4 Index every 2-4 weeks during initial treatment 5
- Maintenance dosing typically ranges from 5-15 mg daily 1
- If TSH becomes suppressed during therapy, reduce or discontinue the dose as this indicates overtreatment or recovery of thyroid function 5
Adjunctive Therapy
Add a beta-blocker (propranolol or atenolol 25-50 mg daily) for symptomatic relief until thyroid hormone levels normalize 6, 5. Beta-blockers should be titrated to maintain heart rate <90 bpm if blood pressure allows 6.
Critical Safety Monitoring
Agranulocytosis is a serious adverse effect that presents with sore throat and fever—obtain immediate complete blood count and discontinue methimazole immediately if these symptoms develop 5. Adverse effects requiring drug discontinuation occur more frequently at 30 mg daily (14.8%) compared to 15 mg daily (7.5%) 4.
Special Population: Pregnancy
In pregnancy, propylthiouracil is preferred over methimazole during the first trimester due to lower risk of birth defects 5. Methimazole may be used in the second and third trimesters if needed.
Pediatric Dosing
For children, the initial daily dose is 0.4 mg/kg body weight divided into 3 doses at 8-hour intervals, with maintenance dosing approximately half of the initial dose 1.