What is the starting dose of methimazole (antithyroid medication) for treating hyperthyroidism?

Determining Starting Dose of Methimazole for Hyperthyroidism

The initial daily dosage of methimazole should be 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

Adult Dosing Algorithm

1. Assess severity of hyperthyroidism:

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

2. Administration schedule:

   • Divide total daily dose into 3 equal portions
   • Administer at 8-hour intervals
   • Take with food to minimize gastrointestinal side effects

3. Maintenance dosing:

   • Once euthyroidism is achieved, reduce to 5-15 mg daily 1
   • Monitor thyroid function every 2-4 weeks initially, then every 3-6 months once stable

Pediatric Dosing

• Initial dose: 0.4 mg/kg body weight daily, divided into 3 doses at 8-hour intervals 1
• Maintenance dose: Approximately half of the initial dose 1

Special Considerations

Pregnancy

For pregnant patients, dosing requires special consideration:

• First trimester: Propylthiouracil (PTU) is preferred due to lower risk of congenital malformations 2
• Second and third trimesters: Switch to methimazole at appropriate dose based on severity 2

Alternative Dosing Approaches

Research suggests that a single daily dose of 15 mg methimazole may be effective for most patients with Graves' hyperthyroidism while causing fewer adverse effects 3. However, the FDA-approved labeling still recommends divided dosing 1, which should be prioritized in clinical practice.

Monitoring and Dose Adjustment

• Measure free T4 or Free T4 Index every 2-4 weeks initially
• Adjust dose based on thyroid function test results
• Target: Free T4 or Free T4 Index in the high-normal range using lowest possible dose
• Watch for signs of agranulocytosis (sore throat, fever), which requires immediate discontinuation and CBC testing

Common Pitfalls to Avoid

1. Overdosing: Using doses higher than 15 mg/day may not significantly increase intrathyroidal concentrations of methimazole 4, but may increase risk of adverse effects.

2. Inadequate monitoring: Failure to monitor thyroid function every 2-4 weeks initially can lead to under or over-treatment.

3. Missing adverse effects: Watch for signs of agranulocytosis, hepatitis, vasculitis, and thrombocytopenia. If these occur, discontinue methimazole immediately.

4. Inappropriate use in pregnancy: Using methimazole in the first trimester increases risk of congenital malformations; PTU is preferred during this period.

By following this dosing algorithm and monitoring protocol, you can effectively manage hyperthyroidism while minimizing the risk of adverse effects.