Methimazole Treatment Initiation Protocol
For adult patients with hyperthyroidism, methimazole should be started at 15 mg daily for mild disease, 30-40 mg daily for moderate disease, and 60 mg daily for severe disease, divided into 3 doses taken at 8-hour intervals. 1
Initial Dosing Guidelines
Adults:
- Mild hyperthyroidism: 15 mg daily (5 mg every 8 hours)
- Moderate hyperthyroidism: 30-40 mg daily (10-13.3 mg every 8 hours)
- Severe hyperthyroidism: 60 mg daily (20 mg every 8 hours)
Children:
- Initial dose: 0.4 mg/kg body weight daily, divided into 3 doses at 8-hour intervals
- Maintenance dose: Approximately half of the initial dose 1
Administration Recommendations
- Take methimazole with food to minimize gastrointestinal side effects 2
- Maintain consistent timing each day for more stable blood levels 2
- Once euthyroid state is achieved, transition to maintenance dosing (typically 5-15 mg daily) 2, 1
Monitoring Protocol
Initial Phase:
- Monitor thyroid function every 2-4 weeks during initial treatment 2
- Aim to maintain free T4 or Free T4 Index in the high-normal range 2
- Most patients (approximately 80%) will become euthyroid within 8 weeks of starting treatment 3
Maintenance Phase:
- Once stable, monitor thyroid function every 3-6 months 2
- Gradually reduce dose to maintenance level (5-15 mg daily) 2, 3
Side Effect Management
Watch for warning signs of serious adverse effects:
- Agranulocytosis: Sore throat, fever
- Hepatotoxicity: Nausea, vomiting, abdominal pain, dark urine
- Other potential side effects: Rash, vasculitis, thrombocytopenia 2
If these symptoms develop, obtain a complete blood count and discontinue methimazole immediately 2
Adjunctive Therapy
- Consider beta-blockers (e.g., propranolol) to control symptoms until methimazole reduces thyroid hormone levels 2
Special Considerations
Pregnancy:
- Propylthiouracil is preferred in the first trimester due to potential teratogenicity with methimazole
- Switch to methimazole in the second and third trimesters due to propylthiouracil-associated hepatotoxicity 2
Treatment Duration:
- Conventional treatment duration is typically 18-24 months
- Longer-term treatment (60-120 months) has shown significantly higher remission rates (85% vs 47%) with good safety profile 4
Efficacy Factors
Several factors may affect response to methimazole treatment:
- Higher initial doses lead to faster normalization of thyroid function
- Larger goiter size may delay response
- Higher pretreatment T3 levels may require longer treatment time 5
Important Caveats
- Methimazole is the preferred antithyroid drug due to its longer half-life and fewer severe side effects compared to propylthiouracil 6
- Single daily dosing (15 mg once daily) has shown similar efficacy to divided dosing (10 mg three times daily) with fewer adverse effects in some studies 3
- No additional adverse effects have been observed with long-term treatment (up to 120 months) beyond those seen in the first 18 months of therapy 4