What is the recommended starting dose of carbimazole (antithyroid medication) for hyperthyroidism?

Starting Dosage of Carbimazole for Hyperthyroidism

For most patients with hyperthyroidism, carbimazole should be initiated at 20 mg once daily, with 40 mg daily reserved only for those with severe hyperthyroidism (baseline total T4 >260 nmol/L). 1

Initial Dosing Strategy

Standard starting dose:

• 20 mg once daily is the recommended initial dose for patients with mild to moderate hyperthyroidism, as this provides effective control while minimizing the risk of iatrogenic hypothyroidism 1
• This dose can be administered as a single daily dose, which is as effective as divided dosing and improves patient adherence 2, 3

Higher initial dose (40 mg daily):

• Reserve for patients with severe hyperthyroidism (baseline total T4 >260 nmol/L or equivalent free T4 elevation) 1
• While 40 mg daily achieves more rapid biochemical control at 4 weeks, it carries significantly higher risk of drug-induced hypothyroidism 1

Dosing Schedule

Once-daily administration is preferred:

• Carbimazole 30 mg given as a single daily dose achieves euthyroidism in comparable timeframes to divided dosing (approximately 3-5 weeks) 2, 3
• The long intrathyroidal half-life of the active metabolite (methimazole) supports once-daily dosing despite shorter plasma half-life 2
• Single daily dosing improves compliance and is particularly useful for patients who struggle with multiple daily doses 2

Clinical Monitoring and Dose Adjustment

Initial response assessment:

• Doses as low as 5-10 mg daily produce marked reduction in intrathyroidal iodide binding, suggesting 10 mg may be effective in some patients 4
• Clinical and biochemical improvement typically occurs within 1-3 months, with patients achieving normal serum T4 levels 3
• Patients with larger goiters and elevated alkaline phosphatase may require longer treatment duration to achieve euthyroidism 3

Key monitoring points:

• Assess thyroid function at 4 weeks to determine if initial dose is adequate 1
• For patients started on 20 mg daily with baseline T4 >260 nmol/L, consider dose escalation if inadequate response at 4 weeks 1
• Adjust dose based on thyroid hormone levels to maintain euthyroidism while avoiding overtreatment 1

Important Caveats

Risk stratification matters:

• The 20 mg starting dose has lower risk of iatrogenic hypothyroidism at both 4 and 10 weeks compared to 40 mg 1
• However, 20 mg may be insufficient for severe hyperthyroidism, requiring upward titration 1
• Starting too low (below 20 mg) may delay achievement of euthyroidism, though doses as low as 10 mg can be effective in selected patients 4

Practical considerations:

• Propranolol can be added for the first 2-3 weeks for symptomatic control while awaiting carbimazole effect 3
• Treatment duration typically ranges from 9-41 weeks, with median around 18 weeks when stopped after achieving low-normal T3/T4 3
• Approximately 39% of patients achieve prolonged remission when treatment is stopped at appropriate biochemical endpoints 3