What is the recommended treatment and dosage for hyperthyroidism using Methimazole?

Methimazole Treatment for Hyperthyroidism

Recommended Dosing

For adults with hyperthyroidism, initiate methimazole at 15 mg daily for mild disease, 30-40 mg daily for moderate disease, and 60 mg daily for severe hyperthyroidism, divided into three doses given at 8-hour intervals. 1

Initial Dosing Strategy by Disease Severity

• Mild hyperthyroidism: Start with 15 mg daily divided into three doses 1
• Moderate hyperthyroidism: Start with 30-40 mg daily divided into three doses 1
• Severe hyperthyroidism: Start with 60 mg daily divided into three doses 1

The maintenance dosage is 5-15 mg daily once thyroid hormone levels normalize. 1

Pediatric Dosing

• Initial dose: 0.4 mg/kg body weight daily, divided into three doses at 8-hour intervals 1
• Maintenance dose: Approximately half of the initial dose 1

Treatment Goals and Monitoring

The primary goal is to maintain free T4 or Free Thyroxine Index (FTI) in the high-normal range using the lowest possible thioamide dosage. 2, 3

Monitoring Schedule

• Check free T4 or FTI every 2-4 weeks to adjust dosing appropriately 2, 3
• Continue monitoring until thyroid hormone levels stabilize in the target range 2

Symptomatic Management

Until methimazole reduces thyroid hormone levels, use a beta-blocker (such as propranolol) to control symptoms including tachycardia, tremor, and anxiety. 2, 3

Evidence for Single Daily Dosing

While the FDA label recommends divided doses, research demonstrates that single daily dosing of 15 mg methimazole achieves euthyroidism in 93% of patients within 12 weeks and causes fewer adverse effects than higher doses. 4 Single daily dosing of 15 mg normalized free T4 in 77% of patients within 6 weeks. 4

For severe hyperthyroidism (free T4 ≥7 ng/dL), methimazole 30 mg daily is more effective than 15 mg daily, achieving normalization of free T4 in 96.5% versus 86.2% of patients at 12 weeks. 5

Critical Adverse Effects

Agranulocytosis

If patients develop sore throat and fever, immediately obtain a complete blood count and discontinue methimazole. 2 This is the most serious adverse effect requiring urgent evaluation.

Other Adverse Effects

• Hepatitis: Monitor liver function tests 2
• Vasculitis: Watch for skin manifestations 2
• Thrombocytopenia: Monitor platelet counts 2

Adverse effects requiring discontinuation occur in 7.5-14.8% of patients, with higher rates at 30 mg daily compared to 15 mg daily. 6, 5

Special Populations

Pregnancy

Pregnant women with Graves' disease can safely use methimazole, maintaining free T4 in the high-normal range with the lowest effective dose. 2 Monitor every 2-4 weeks until stable, then every trimester. 2

• Fetal thyroid suppression may occur but is usually transient and rarely requires treatment 2
• Women can safely breastfeed while taking methimazole 2
• Radioactive iodine (I-131) is absolutely contraindicated in pregnancy 2

Thyroid Storm

In thyroid storm, use methimazole as part of a multi-drug regimen including saturated solution of potassium iodide or sodium iodide, dexamethasone, and supportive measures. 2 Treatment should not be delayed for laboratory confirmation given the high mortality risk. 2

Factors Affecting Treatment Response

Response to methimazole is primarily determined by the daily dose, pretreatment T3 levels, and goiter size. 7

• Larger goiters require longer time to achieve euthyroidism 7
• Higher pretreatment thyroid hormone levels predict delayed response 7
• Urinary iodine excretion ≥100 micrograms/g creatinine delays response 7