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

Methimazole Treatment for Hyperthyroidism

For hyperthyroidism treatment, methimazole should be administered at an initial daily dose of 15 mg for mild disease, 30-40 mg for moderate disease, and 60 mg for severe disease, typically given as a single daily dose. 1

Dosing Guidelines

Initial Dosing

• For adults with mild hyperthyroidism: 15 mg daily 1
• For adults with moderately severe hyperthyroidism: 30-40 mg daily 1
• For adults with severe hyperthyroidism: 60 mg daily 1
• For pediatric patients: 0.4 mg/kg of body weight daily 1

Administration

• While the FDA label recommends dividing the total daily dose into three equal portions given at 8-hour intervals 1, more recent evidence supports using methimazole as a single daily dose due to its prolonged intrathyroidal effect 2, 3
• Single daily dosing has shown equal efficacy to divided doses with potentially fewer side effects 2
• Intrathyroidal concentrations of methimazole are maintained for at least 26 hours, supporting once-daily administration 3

Maintenance Dosing

• Adult maintenance dose: 5-15 mg daily 1
• Pediatric maintenance dose: Approximately half of the initial dose 1
• Dose adjustments should be made based on thyroid function tests, typically every 4-6 weeks 4

Monitoring

• TSH and Free T4 should be used for case detection and monitoring in symptomatic patients 4
• For asymptomatic patients on therapy, TSH can be checked every 4-6 weeks 4
• After initiating treatment, monitor thyroid function every 2-3 weeks to catch potential transition to hypothyroidism 4
• T3 levels can be helpful in highly symptomatic patients with minimal FT4 elevations 4

Special Considerations

Pregnancy

• Both methimazole and propylthiouracil (PTU) have similar therapeutic efficacy and placental transfer kinetics 5
• However, methimazole use in pregnancy has been associated with rare congenital anomalies (aplasia cutis and choanal/esophageal atresia) 5
• Therefore, PTU is generally preferred during pregnancy, particularly in the first trimester 5
• Women treated with methimazole can safely breastfeed 4, 5

Graves' Disease

• Consider TSH receptor antibody testing if there are clinical features suggestive of Graves' disease (e.g., ophthalmopathy, T3 toxicosis) 4
• Physical examination findings of ophthalmopathy or thyroid bruit are diagnostic of Graves' disease and should prompt early endocrine referral 4
• Graves' disease is generally persistent and may require longer-term therapy 4

Thyroid Storm

• In severe cases or thyroid storm, hospitalization is recommended 4
• Treatment includes methimazole or propylthiouracil, beta-blockers, potassium iodide solutions, and supportive care 4
• General supportive measures including oxygen, antipyretics, and appropriate monitoring are also important 4

Side Effects and Safety

• Major side effects include agranulocytosis, hepatitis, vasculitis, and thrombocytopenia 5
• Agranulocytosis typically presents with sore throat and fever - if these develop, obtain a complete blood count and discontinue the medication 4
• Single daily dosing may be associated with fewer adverse effects compared to divided dosing 2
• Research has shown that doses above 15 mg/day do not significantly increase intrathyroidal concentrations of methimazole, suggesting 15 mg/day may be the most satisfactory dose for treating hyperthyroidism in many cases 3

Adjunctive Treatments

• Beta-blockers (e.g., atenolol or propranolol) can be used for symptomatic relief while waiting for methimazole to take effect 4
• Hydration and supportive care are important components of management, especially in moderate to severe cases 4