Starting Dose of Methimazole for Hyperthyroidism
The initial daily dosage of methimazole for treating 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
The appropriate starting dose of methimazole (MMI) depends primarily on the severity of hyperthyroidism:
- Mild hyperthyroidism: 15 mg daily
- Moderate hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily
These doses are typically divided into three equal portions given at 8-hour intervals according to the FDA-approved labeling 1.
Administration Considerations
Dosing Schedule Options
While the standard recommendation is to divide the daily dose into three equal portions, research has shown that a single daily dose can be effective for many patients:
- Studies have demonstrated that a single daily dose of 15 mg MMI can effectively control hyperthyroidism in most patients with Graves' disease 2
- Single daily dosing may improve medication adherence and reduce side effects compared to multiple daily doses 2
Monitoring and Dose Adjustment
After initiating therapy:
- Monitor free T4 or free T3 index (FTI) every 2-4 weeks initially
- The goal is to maintain free T4 or FTI in the high-normal range using the lowest possible thioamide dosage 3
- Adjust dose based on clinical response and laboratory values
- Maintenance dose typically ranges from 5-15 mg daily 1
Special Populations
Pregnant Women
For pregnant women with hyperthyroidism:
- Methimazole or propylthiouracil can be used
- The goal is to use the lowest effective dose to maintain FT4 or FTI in the high-normal range 3
- Regular monitoring every 2-4 weeks is recommended 3
Pediatric Patients
For children with hyperthyroidism:
- Initial daily dose: 0.4 mg/kg of body weight divided into 3 doses given at 8-hour intervals
- Maintenance dose: Approximately half of the initial dose 1
Side Effects and Monitoring
Common Side Effects
- Agranulocytosis (presents with sore throat and fever)
- Hepatitis
- Vasculitis
- Thrombocytopenia 3
Monitoring Recommendations
- If symptoms of agranulocytosis develop (sore throat and fever), obtain a complete blood count and discontinue methimazole 3
- Monitor thyroid function tests regularly to adjust dosing as needed
Treatment Considerations
Comparative Efficacy
Research has shown that MMI 30 mg/day normalizes free T4 more effectively than propylthiouracil (PTU) 300 mg/day, particularly in patients with severe hyperthyroidism (FT4 ≥7 ng/dl) 4.
Long-term Management
For maintenance therapy after achieving euthyroidism:
- Typical maintenance dose: 5-15 mg daily 1
- Some evidence suggests that long-term continuation of low-dose MMI (2.5-5 mg daily) may prevent recurrence of hyperthyroidism 5
Adjunctive Therapy
Until methimazole reduces thyroid hormone levels, a beta blocker (e.g., propranolol) can be used to control symptoms 3.
By following these dosing guidelines and monitoring protocols, methimazole therapy can effectively manage hyperthyroidism while minimizing the risk of adverse effects.