Methimazole Treatment for Hyperthyroidism
For adult hyperthyroidism, methimazole should be initiated at 15 mg daily for mild disease, 30-40 mg daily for moderate disease, and 60 mg daily for severe disease, divided into three doses at 8-hour intervals, with maintenance dosing of 5-15 mg daily. 1
Initial Dosing Strategy
The FDA-approved dosing for methimazole follows a severity-based approach:
- Mild hyperthyroidism: 15 mg daily
- Moderate hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily
These doses should initially be divided into three equal doses given at 8-hour intervals to maximize efficacy 1.
Pediatric Dosing
- Initial dose: 0.4 mg/kg/day divided into 3 doses at 8-hour intervals
- Maintenance dose: Approximately half of the initial dose 1
Treatment Monitoring and Adjustment
- Monitor free T4 or free T4 index (FTI) every 2-4 weeks during initial treatment 2
- Goal is to maintain FT4 or FTI in the high-normal range using the lowest possible methimazole dosage 2
- Once stabilized, the maintenance dose typically ranges from 5-15 mg daily 1
Adjunctive Therapy
- Beta blockers (e.g., propranolol) can be used to control symptoms until methimazole reduces thyroid hormone levels 2, 3
- For thyroid storm (severe hyperthyroidism with systemic decompensation), additional treatments include:
- Saturated solution of potassium iodide or sodium iodide
- Dexamethasone
- Supportive care (oxygen, antipyretics, monitoring) 3
Efficacy Considerations
Research shows that higher initial doses normalize thyroid function more quickly:
- In patients with severe hyperthyroidism (FT4 ≥7 ng/dL), 30 mg/day normalizes FT4 more effectively than 15 mg/day 4
- For mild to moderate hyperthyroidism, 15 mg/day may be sufficient 4
Administration Options
While the FDA label recommends divided doses, research indicates that a single daily dose may be effective:
- A study found that 93% of patients treated with a single daily dose of 15 mg became euthyroid within 12 weeks 5
- The mean time to achieve euthyroidism was approximately 5.3 weeks 5
Adverse Effects and Monitoring
Common adverse effects include:
- Agranulocytosis (presents with sore throat and fever)
- Hepatitis
- Vasculitis
- Thrombocytopenia 2
If symptoms of agranulocytosis develop (sore throat and fever), obtain a complete blood count and discontinue methimazole immediately 2.
Special Populations
Pregnancy
- Both propylthiouracil and methimazole can be used in pregnancy 2
- Women treated with methimazole can breastfeed safely 2
- Monitor fetal heart rate and growth in pregnant women with Graves' disease 2
Severe Cases and Thyroid Storm
For thyroid storm, a life-threatening condition, immediate treatment includes:
- Methimazole or propylthiouracil
- Beta blockers
- Iodide solutions
- Corticosteroids
- Supportive care 3
Treatment Duration and Maintenance
Long-term therapy with low-dose methimazole (as low as 1.25-2.5 mg daily) has been shown to be effective and safe for maintaining euthyroidism in some patients 6.
Important Caveats
- Monitoring for agranulocytosis: Patients should be instructed to report sore throat and fever immediately
- Hepatotoxicity risk: Higher with propylthiouracil than with methimazole 4
- Iodine intake: Higher urinary iodide excretion (>100 μg/g creatinine) may delay response to methimazole 7
- Goiter size: Larger goiters may require higher doses or longer treatment 7
For patients who do not respond adequately to methimazole therapy, thyroidectomy should be considered as an alternative treatment option 2.