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

Methimazole Treatment for Hyperthyroidism

For hyperthyroidism, 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, typically divided into 3 doses at 8-hour intervals, followed by a maintenance dosage of 5-15 mg daily. 1

Dosing Regimen

Initial Dosing

• Mild hyperthyroidism: 15 mg daily
• Moderate hyperthyroidism: 30-40 mg daily
• Severe hyperthyroidism: 60 mg daily 1

Administration

• Initially divided into 3 doses at 8-hour intervals
• Single daily dosing may be effective for many patients, with comparable efficacy to divided dosing but potentially fewer side effects 2

Maintenance Dosing

• Once euthyroid state is achieved, reduce to maintenance dose of 5-15 mg daily 1
• For pediatric patients: Initial dose of 0.4 mg/kg/day divided into 3 doses, with maintenance dose approximately half of the initial dose 1

Monitoring and Dose Adjustment

• Measure Free T4 or Free T4 Index every 2-4 weeks during initial treatment 3
• Goal: Maintain Free T4 or Free T4 Index in the high-normal range using the lowest possible thioamide dosage 3
• Continue monitoring thyroid function (TSH and Free T4) every 4-6 weeks initially, then every 6-12 months once stable 4

Adjunctive Therapy

• Beta-blockers (e.g., propranolol 60-80 mg orally every 4-6 hours) can be used to control symptoms until methimazole reduces thyroid hormone levels 3, 4
• Discontinue beta-blockers once thyroid hormone levels normalize

Special Considerations

Pregnancy

• Methimazole can be used during pregnancy, though some clinicians prefer propylthiouracil in the first trimester 3
• Women treated with methimazole can breastfeed safely 3

Thyroid Storm

• For thyroid storm (a life-threatening emergency), treatment includes:
  • Methimazole or propylthiouracil
  • Saturated solution of potassium iodide or sodium iodide
  • Dexamethasone
  • Beta-blockers (propranolol 60-80 mg every 4-6 hours or 1-2 mg IV slowly)
  • Supportive care (oxygen, antipyretics, monitoring) 4

Side Effects and Monitoring

Common Side Effects

• Rash, urticaria, arthralgia, fever

Serious Adverse Effects

• Agranulocytosis: Presents with sore throat and fever; requires immediate CBC and drug discontinuation 3
• Other serious effects: Hepatitis, vasculitis, thrombocytopenia 3

Monitoring for Adverse Effects

• Instruct patients to report sore throat, fever, or other signs of infection immediately
• Consider periodic CBC monitoring, especially during the first months of therapy

Duration of Treatment

• Standard treatment duration is typically 12-18 months
• Risk of relapse is high (approximately 50%) after discontinuation
• Long-term low-dose therapy (1.25-2.5 mg daily) may be effective and safe for maintaining euthyroidism in patients with multiple relapses 5

Alternative Approaches

• For moderate to severe hyperthyroidism, combining lower-dose methimazole (15 mg/day) with inorganic iodine (38 mg/day) may achieve euthyroidism more rapidly with fewer adverse effects than methimazole 30 mg/day alone 6
• Thyroidectomy should be reserved for patients who do not respond to thioamide therapy 3
• Radioactive iodine (I-131) is contraindicated in pregnant women 3

Remember that the goal of treatment is to control hyperthyroidism symptoms and normalize thyroid hormone levels while minimizing side effects. Regular monitoring is essential to adjust dosing appropriately and detect any adverse effects early.