Methimazole Dosing for Hyperthyroidism
The typical dosage of Methimazole (MMI) for treating hyperthyroidism is 15 mg daily for mild hyperthyroidism, 30-40 mg daily for moderately severe hyperthyroidism, and 60 mg daily for severe hyperthyroidism, with doses typically divided into three administrations at 8-hour intervals. 1
Initial Dosing Strategy
- For mild hyperthyroidism: Start with 15 mg daily 1
- For moderate hyperthyroidism: Start with 30-40 mg daily 1
- For severe hyperthyroidism: Start with 60 mg daily 1
- Doses are typically divided into three equal portions given at 8-hour intervals, though some evidence supports once-daily dosing 2
Factors Affecting Dosing Decisions
- Severity of hyperthyroidism as determined by free T4 and T3 levels 3
- Goiter size (larger goiters may require higher doses) 3
- Patient age (lower doses may be preferred in elderly patients) 4
- Iodine status (higher iodine intake may delay response to treatment) 3
Maintenance Therapy
- After initial control of hyperthyroidism, maintenance dosage is typically 5-15 mg daily 1
- Some patients benefit from long-term continuation of low-dose MMI (2.5-5 mg daily) to prevent recurrence 5
- Monitor thyroid function every 2-4 weeks initially to adjust dosing appropriately 6
Special Populations
Pediatric Patients
- Initial dose: 0.4 mg/kg body weight daily, divided into 3 doses 1
- Maintenance dose: Approximately half of the initial dose 1
- Lower doses (<0.7 mg/kg/day) are associated with fewer adverse events in children 4
Pregnant Women
- MMI can be used in pregnancy, with the goal of maintaining free T4 or Free T4 Index in the high-normal range using the lowest possible dose 6
- Monitor thyroid function every 2-4 weeks during pregnancy 6
Immune Checkpoint Inhibitor-Induced Hyperthyroidism
- For severe symptoms from immune-related thyroiditis, MMI may be used as part of management 6
- Consult endocrinology for guidance on dosing in these cases 6
Monitoring and Dose Adjustment
- Measure free T4 and TSH every 2-4 weeks initially until stable 6
- Adjust dose based on thyroid function tests to achieve normal thyroid hormone levels 1
- Once stable, monitor every 3 months in the first year, then every 6 months 6
Adverse Effects to Monitor
- Agranulocytosis (presents with sore throat and fever) - requires immediate discontinuation and CBC 6
- Rash and neutropenia can occur at any dose, including low doses 4
- Other potential adverse effects include hepatitis, vasculitis, and thrombocytopenia 6
- Higher doses (≥30 mg/day) are associated with more frequent adverse effects 7
Alternative Dosing Approaches
- Single daily dosing of 15 mg MMI has shown similar efficacy to divided doses with potentially fewer side effects 2
- Combination therapy with 15 mg MMI plus inorganic iodine (38 mg/day) may achieve faster normalization of thyroid function than 30 mg MMI alone 7
Clinical Pearls
- Response to MMI is typically seen within 3-6 weeks of starting therapy 3, 2
- Patients with large goiters and higher pretreatment T3 levels may have a delayed response 3
- Beta-blockers (e.g., propranolol) may be added for symptomatic relief until MMI reduces thyroid hormone levels 6
- In patients with thyroiditis, hyperthyroidism is often self-limited and may resolve in weeks with supportive care 6