Methimazole Dosage for Hyperthyroidism Treatment
The recommended initial daily dosage of methimazole for treating hyperthyroidism is 15 mg for mild hyperthyroidism, 30-40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, typically divided into 3 doses at 8-hour intervals, with maintenance dosage of 5-15 mg daily. 1
Initial Dosing Based on Disease Severity
The appropriate dosing of methimazole depends on the severity of hyperthyroidism:
- Mild hyperthyroidism: 15 mg daily
- Moderate hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily 1
Administration Schedule
While the FDA label recommends dividing the total daily dose into three equal doses given at 8-hour intervals 1, research evidence suggests that a single daily dose can be equally effective:
- Studies have demonstrated that a single daily dose of 15 mg methimazole is effective in controlling hyperthyroidism in most patients 2, 3
- Single daily dosing may be associated with fewer adverse effects compared to divided doses (13% vs 24%) 3
- Intrathyroidal concentrations of methimazole are maintained for at least 26 hours, supporting once-daily dosing 4
Maintenance Dosing
After achieving euthyroidism (normal thyroid function):
- Reduce to maintenance dose of 5-15 mg daily 1
- Typical maintenance dose is approximately half of the initial dose
- Monitor thyroid function every 4-6 weeks initially, then every 3-6 months once stable 5
Special Populations
Pediatric Patients
- Initial dose: 0.4 mg/kg body weight daily, divided into 3 doses given at 8-hour intervals
- Maintenance dose: Approximately half of the initial dose 1
Pregnancy
- Propylthiouracil is preferred in the first trimester due to possible teratogenicity with methimazole
- Methimazole is preferred in the second and third trimesters due to propylthiouracil-associated hepatotoxicity 6
- Goal is to maintain free T4 or Free T4 Index in the high-normal range using the lowest possible thioamide dosage 6
Monitoring and Follow-up
- Measure free T4 or Free T4 Index every 2-4 weeks during initial treatment 6
- Monitor for side effects, particularly agranulocytosis, which presents with sore throat and fever 6
- Other potential side effects include hepatitis, vasculitis, and thrombocytopenia 6
- If agranulocytosis symptoms develop, obtain a complete blood count and discontinue methimazole 6
Important Considerations
- Research suggests that doses above 15 mg/day may not significantly increase intrathyroidal concentrations of methimazole, indicating a saturable uptake mechanism 4
- A combination of lower-dose methimazole (15 mg) with inorganic iodine may achieve faster normalization of thyroid function with fewer adverse effects compared to higher-dose methimazole (30 mg) alone 7
- Beta-blockers (e.g., propranolol) can be used to reduce symptoms until methimazole reduces thyroid hormone levels 6
Remember to monitor patients closely for adverse effects, particularly during the initial treatment phase, and adjust dosing based on clinical response and laboratory values.