Initial Dosage of Methimazole for Hyperthyroidism
The initial daily dosage of methimazole for hyperthyroidism is 15 mg for mild hyperthyroidism, 30 mg to 40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. 1
Dosing Based on Disease Severity
Mild Hyperthyroidism
- Initial dose: 15 mg daily 1
- Can be administered as a single daily dose or divided into three doses 2
- Single daily dosing of 15 mg has been shown to be effective in controlling hyperthyroidism in most patients while causing fewer adverse effects than higher doses 2
Moderate to Severe Hyperthyroidism
- Initial dose: 30-40 mg daily for moderately severe cases 1
- Initial dose: 60 mg daily for severe cases 1
- Traditionally divided into three equal doses given at 8-hour intervals 1
- Alternative regimen for moderate to severe cases: 15 mg methimazole plus inorganic iodine (38 mg/day), which may achieve euthyroidism faster with fewer adverse effects than 30 mg methimazole alone 3
Administration Considerations
- While the FDA label recommends divided dosing, research has demonstrated that single daily dosing is effective for most patients 4, 2
- Single daily dosing of 15 mg has shown similar efficacy to divided doses (10 mg three times daily) in achieving euthyroidism 5
- Mean time to achieve euthyroidism with appropriate dosing is approximately 5-6 weeks 2, 5
- The maintenance dose is typically 5-15 mg daily after achieving euthyroid status 1
Monitoring and Dose Adjustment
- Monitor thyroid function tests (TSH, FT4) every 2-4 weeks initially to assess response 6
- For persistent hyperthyroidism beyond 6-8 weeks, consider endocrine consultation for potential dose adjustment 6
- Once euthyroid status is achieved, gradually reduce the dose to a maintenance level (typically 5 mg daily) 5
Adverse Effects and Precautions
- Higher initial doses (30 mg vs 15 mg) are associated with increased risk of adverse effects 2
- Common adverse effects include:
- Patients should be instructed to report symptoms of agranulocytosis (sore throat, fever) immediately and discontinue the medication if these occur 6
- For severe hyperthyroidism with significant symptoms, beta-blockers (e.g., propranolol or atenolol) may be added for symptomatic relief until methimazole takes effect 6
Special Populations
- Pediatric patients: Initial daily dosage is 0.4 mg/kg of body weight divided into 3 doses at 8-hour intervals, with maintenance dosage approximately half of the initial dose 1
- Pregnant women: Methimazole can be used during pregnancy, with the goal of maintaining FT4 or FTI in the high-normal range using the lowest possible dosage 6
- Elderly patients or those with cardiovascular disease may require more careful dosing and monitoring due to increased risk of adverse effects 6