Methimazole Dosing for Mild Hyperthyroidism
Direct Answer
Start methimazole 15 mg daily (given as a single daily dose) for this patient with mild hyperthyroidism. 1
Assessment of Disease Severity
Your patient's laboratory values indicate mild hyperthyroidism:
- TSH 0.09 mIU/L (suppressed, confirming hyperthyroidism) 2
- T4 0.73 (mildly elevated, assuming normal range ~0.5-0.7)
- T3 3.4 (mildly elevated)
The degree of thyroid hormone elevation, rather than TSH suppression alone, determines disease severity and initial methimazole dosing 1, 3.
FDA-Approved Dosing Algorithm
The FDA label provides clear stratification 1:
- Mild hyperthyroidism: 15 mg daily
- Moderately severe hyperthyroidism: 30-40 mg daily
- Severe hyperthyroidism: 60 mg daily
Your patient's mildly elevated T3 and T4 levels place them squarely in the mild category, warranting the 15 mg starting dose 1.
Single Daily Dosing vs. Divided Dosing
Administer the entire 15 mg dose once daily rather than dividing it, despite the FDA label mentioning divided doses 1. Research demonstrates that single daily dosing (15 mg once) achieves euthyroidism in the same timeframe as conventional divided dosing (10 mg three times daily), with significantly fewer adverse effects (13% vs 24%) 4. The mean time to achieve euthyroid status is approximately 5.8 weeks with either regimen 4.
Expected Response Timeline
- Within 3 weeks: Approximately 40-46% of patients on 15 mg will achieve euthyroidism (higher percentage if iodine intake is low) 3
- Within 6 weeks: Approximately 77-80% will be euthyroid 3, 4
- By 8 weeks: Nearly 80% achieve control 4
The main determinants of response speed are the initial methimazole dose, pretreatment T3 levels, and goiter size 3.
Monitoring Protocol
- Recheck thyroid function (TSH, free T4, free T3) at 3 weeks, then at 6 weeks 3
- Once euthyroid, gradually reduce to maintenance dose of 5 mg daily 1, 4
- Continue monitoring every 6-8 weeks during dose titration 5
Critical Safety Considerations
Agranulocytosis occurs in 0.1-0.5% of patients and represents the most dangerous adverse effect 6. Instruct the patient to immediately report fever, sore throat, or signs of infection. Other adverse effects (rash, mild leukopenia, liver dysfunction) occur in 3-5% of patients but are typically minor and transient 6.
Common Pitfalls to Avoid
- Do not start with 30-40 mg for mild disease—this increases adverse effects without improving outcomes and risks overtreatment 1, 3
- Do not use divided dosing—single daily dosing reduces adverse effects by nearly 50% with equivalent efficacy 4
- Do not delay monitoring beyond 6 weeks—early assessment allows timely dose adjustment and prevents prolonged hyperthyroidism 3
- Do not maintain the initial 15 mg dose indefinitely—reduce to 5 mg maintenance once euthyroid to minimize long-term adverse effects 1, 4