What is the recommended dose of carbimazole (antithyroid medication) for treating hyperthyroidism in adults?

Carbimazole Dosing for Hyperthyroidism

For adults with hyperthyroidism, start carbimazole at 20-40 mg daily (given as a single daily dose), with 20 mg/day preferred for mild-to-moderate disease and 40 mg/day reserved for severe hyperthyroidism (baseline T4 >260 nmol/L), then titrate to the lowest effective dose to maintain free T4 in the high-normal range. 1, 2

Initial Dosing Strategy

Single daily dosing is as effective as divided doses and improves adherence:

• 20 mg daily is the preferred starting dose for most patients with mild-to-moderate hyperthyroidism, as it effectively controls thyroid hormone levels while minimizing the risk of iatrogenic hypothyroidism 1
• 40 mg daily should be reserved for patients with severe hyperthyroidism (baseline total T4 >260 nmol/L), as lower doses are less effective in this population 1
• 30 mg daily as a single dose is an alternative effective regimen, typically achieving euthyroidism within 1-3 months 3

Single daily dosing (given at bedtime or with meals) is equally effective as divided doses due to carbimazole's long intrathyroidal half-life, despite its short plasma half-life 4, 3

Monitoring and Dose Adjustment

Monitor free T4 or Free T4 Index every 2-4 weeks during initial treatment:

• The goal is to maintain free T4 in the high-normal range using the lowest possible dose 2
• Patients typically achieve euthyroidism within 3-6 weeks, though those with larger goiters or elevated alkaline phosphatase may take longer 3, 4
• If TSH becomes suppressed during therapy, this indicates overtreatment—reduce or discontinue the dose with close follow-up 2

Comparative Efficacy by Dose

Lower doses reduce risk of iatrogenic hypothyroidism:

• At 4 weeks, 40 mg/day produces significantly lower thyroid hormone levels than 20 mg/day (total T4: 98 vs 158 nmol/L), but clinical responses at 6-12 weeks are similar 1
• Drug-related hypothyroidism is less likely with 20 mg/day at both 4 and 10 weeks compared to 40 mg/day 1
• Even doses as low as 5-10 mg daily produce marked reduction in intrathyroidal iodide binding, suggesting 10 mg may be effective as a starting dose in some patients 5

Adjunctive Therapy

Add a beta-blocker for symptomatic relief until thyroid hormone levels normalize:

• Propranolol or atenolol can be used to reduce symptoms (tachycardia, tremor, anxiety) during the initial weeks of treatment 2
• Beta-blockers are typically needed only for the first 2-4 weeks until antithyroid drug effects become apparent 3

Duration and Remission

Treatment duration typically ranges from 12-18 months:

• In one study using 30 mg daily, median treatment duration was 18 weeks (range 9-41 weeks), with 39% of patients achieving prolonged remission (>1 year) after stopping therapy 3
• Stopping carbimazole when T3 and T4 are in the low-normal range (usually 2-4 months after clinical euthyroidism) may optimize remission rates 3
• Adding thyroxine to carbimazole does not reduce recurrence rates and is not recommended 6

Safety Monitoring

Agranulocytosis is a serious adverse effect requiring immediate action:

• If patients develop sore throat and fever, obtain an immediate complete blood count and discontinue carbimazole 2
• This potentially life-threatening complication typically presents early in treatment

Special Populations

Carbimazole is contraindicated in pregnancy:

• Propylthiouracil (PTU) is preferred over carbimazole/methimazole during the first trimester due to lower risk of birth defects 7, 2
• Carbimazole is metabolized to methimazole, which carries teratogenic risk