What is the initial medication for hyperthyroidism?

Initial Medication for Hyperthyroidism

Methimazole is the first-line antithyroid medication for hyperthyroidism, with an initial dose of 15 mg daily for mild disease, 30-40 mg daily for moderate disease, and 60 mg daily for severe disease, divided into three doses at 8-hour intervals. 1

Primary Treatment Selection

• Methimazole is preferred over propylthiouracil because it has fewer major side effects, can be administered as a single daily dose, is less expensive, and is more widely available 2
• Propylthiouracil should be reserved only for specific situations: first trimester of pregnancy or in patients who have experienced adverse reactions to methimazole 3
• The American Academy of Family Physicians recommends antithyroid medications (such as methimazole) as the principal treatment for hyperthyroidism 4

Initial Dosing Strategy

For adults, the FDA-approved initial dosing is: 1

• Mild hyperthyroidism: 15 mg daily
• Moderately severe hyperthyroidism: 30-40 mg daily
• Severe hyperthyroidism: 60 mg daily

Critical dosing consideration: The starting dose of methimazole should not exceed 15-20 mg/day to minimize the risk of agranulocytosis, which is dose-dependent 3. This creates a clinical tension with FDA labeling for moderate-to-severe disease, but safety data supports lower initial dosing.

Adjunctive Symptomatic Management

• Beta-blockers should be added for symptomatic relief in patients with mild to moderate hyperthyroidism 5
• The American College of Cardiology recommends titrating beta-blocker dose to achieve heart rate <90 bpm if blood pressure allows 5
• Atenolol 25-50 mg daily is preferred due to cardioselectivity 5

Initial Monitoring Protocol

During the first weeks of treatment: 6, 5

• Check free T4 or free thyroxine index every 2-4 weeks during the initial treatment phase
• Adjust methimazole dose based on thyroid function tests to maintain free T4 in the high-normal range
• Continue frequent monitoring until biochemically stable, then extend intervals to every 6-12 months 6

Critical Safety Warnings

Agranulocytosis is the most serious adverse effect, typically presenting with sore throat and fever 6:

• If these symptoms develop, obtain an immediate complete blood count
• Discontinue methimazole immediately if agranulocytosis is confirmed 6
• Other serious adverse effects include hepatitis, vasculitis, and thrombocytopenia 6

Propylthiouracil carries severe hepatotoxicity risk, potentially leading to liver transplantation or death, which is why it should not be used as first-line therapy 3

Special Clinical Scenarios

For severe hyperthyroidism: 5

• Endocrine consultation is recommended
• Hospitalization may be necessary for severe cases with medically significant consequences

For pregnancy: 6

• Both methimazole and propylthiouracil are effective with no significant differences in neonatal outcomes
• Use the lowest possible dose to maintain free T4 in the high-normal range
• Propylthiouracil is preferred in first trimester due to methimazole's association with aplasia cutis and choanal/esophageal atresia 2

Maintenance Phase Transition

• The maintenance dosage is typically 5-15 mg daily once euthyroid state is achieved 1
• The American Academy of Family Physicians recommends maintaining free T4 in the high-normal range using the lowest possible dose 6
• TSH normalization may lag behind T4 normalization by several weeks to months 6