What is the recommended starting dose of methimazole (MMI) for treating hyperthyroidism?

Methimazole Dosing for Hyperthyroidism

The recommended starting dose of methimazole (MMI) for treating hyperthyroidism in adults is 15 mg daily for mild hyperthyroidism, 30-40 mg daily for moderately severe hyperthyroidism, and 60 mg daily for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. 1

Dosing Guidelines Based on Disease Severity

Adult Dosing:

• Mild hyperthyroidism: 15 mg daily
• Moderate hyperthyroidism: 30-40 mg daily
• Severe hyperthyroidism: 60 mg daily

Pediatric Dosing:

• Initial dose: 0.4 mg/kg of body weight daily, divided into 3 doses
• Maintenance dose: Approximately half of the initial dose

Administration Considerations

The FDA-approved label recommends dividing the total daily dose into 3 equal portions given at 8-hour intervals 1. However, research evidence suggests that a single daily dose regimen may be equally effective and associated with fewer adverse effects:

• A study comparing 15 mg once daily versus conventional 10 mg three times daily showed similar efficacy in achieving euthyroidism (mean time 5.8 vs 5.6 weeks) 2
• Single daily dosing had fewer adverse effects (13%) compared to divided dosing (24%) 2
• Another study found that 93% of patients treated with 15 mg single daily dose achieved euthyroidism within 12 weeks 3

Special Considerations in Pregnancy

For women of reproductive age who may become pregnant, propylthiouracil is preferred in the first trimester, while methimazole is preferred in the second and third trimesters 4. This recommendation is based on:

• Possible teratogenicity associated with methimazole in the first trimester
• Propylthiouracil-associated hepatotoxicity in subsequent trimesters

Alternative Treatment Approaches

Research suggests that combining lower doses of methimazole with inorganic iodine may be effective:

• A combination of MMI 15 mg + inorganic iodine 38 mg/day showed faster normalization of thyroid function compared to MMI 30 mg/day alone 5
• This combination approach had fewer adverse effects requiring discontinuation (7.5% vs 14.8%) 5

Monitoring and Dose Adjustment

• After achieving euthyroidism, the maintenance dose is typically 5-15 mg daily 1
• Regular monitoring of thyroid function tests is essential to adjust dosing
• Adverse effects are more common with higher doses, so using the lowest effective dose is recommended

Common Pitfalls to Avoid

1. Inappropriate dose selection: Not adjusting the initial dose based on disease severity can lead to delayed response or increased risk of side effects
2. Overlooking pregnancy considerations: Failing to switch between propylthiouracil and methimazole at appropriate trimesters
3. Inadequate monitoring: Not following thyroid function tests regularly to adjust dosing
4. Medication adherence issues: Not discussing the option of single daily dosing when appropriate, which may improve compliance

By selecting the appropriate initial dose based on disease severity and patient characteristics, methimazole therapy can effectively control hyperthyroidism while minimizing adverse effects.