Methimazole Treatment for Hyperthyroidism
For hyperthyroidism treatment, methimazole should be administered at an initial daily dosage of 15 mg for mild hyperthyroidism, 30-40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals, with maintenance dosage of 5-15 mg daily. 1
Initial Dosing Guidelines
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 1
These doses are typically divided into three equal doses given at 8-hour intervals, although some evidence supports once-daily dosing as effective 2, 3.
Administration and Monitoring
- Administer methimazole orally, initially in divided doses
- Monitor thyroid function (TSH and free T4) every 2-4 weeks during initial treatment 4
- Maintain free T4 or Free T4 Index (FTI) in the high-normal range using the lowest possible methimazole dosage 4
- Once stabilized, gradually reduce to maintenance dose of 5-15 mg daily 1
Special Populations
Pregnancy
- Methimazole can be used during pregnancy, though propylthiouracil is often preferred in the first trimester 4
- Women treated with methimazole can breastfeed safely 4
- Monitor TSH every 6-8 weeks during pregnancy
Pediatric Patients
- Initial daily dose: 0.4 mg/kg body weight divided into 3 doses at 8-hour intervals
- Maintenance dose: Approximately half of the initial dose 1
- Long-term methimazole treatment (96-120 months) has shown high cure rates (88-92%) in juvenile Graves' disease 5
Treatment Response Factors
Several factors affect response to methimazole treatment:
- Dose: Higher doses (40 mg vs. 10 mg) achieve euthyroidism more quickly 6
- Goiter size: Larger goiters delay response time
- Pretreatment T3 levels: Higher levels delay normalization
- Iodine intake: Higher urinary iodide excretion (>100 μg/g creatinine) delays response 6
Alternative Dosing Strategies
Research suggests that single daily dosing may be effective:
- A single 15 mg daily dose achieved euthyroidism in 93% of patients within 12 weeks 2
- Single daily dosing of 15 mg methimazole was more effective than 150 mg propylthiouracil once daily 3
- For moderate to severe hyperthyroidism, combining 15 mg methimazole with inorganic iodine (38 mg/day) showed faster normalization of thyroid function with fewer adverse effects compared to 30 mg methimazole alone 7
Adverse Effects and Monitoring
- Common side effects include skin reactions, hepatitis, vasculitis, and thrombocytopenia 4
- Agranulocytosis is a serious side effect that typically presents with sore throat and fever
- If these symptoms develop, obtain a complete blood count and discontinue methimazole 4
- Higher doses (30 mg vs. 15 mg) are associated with more frequent adverse effects 7, 2
- Monitor for symptoms of hypothyroidism during treatment
Treatment Duration
- Typical treatment duration is 12-18 months
- Long-term treatment (up to 96-120 months) may be considered in some cases, particularly in juvenile Graves' disease, as it shows higher remission rates compared to short-term therapy 5