What is the maximum dose of carbimazole (antithyroid medication)?

Maximum Dose of Carbimazole for Hyperthyroidism

The maximum daily dose of carbimazole is 60 mg, typically used as initial therapy for hyperthyroidism, with subsequent dose reduction to the lowest effective maintenance dose (often 5-10 mg daily). 1

Initial Dosing

• Initial doses of carbimazole typically range from 30-60 mg daily depending on the severity of hyperthyroidism 2, 1
• For mild to moderate hyperthyroidism, 20 mg daily is often effective and has a lower risk of iatrogenic hypothyroidism 3
• For severe hyperthyroidism (baseline total T4 > 260 nmol/l), higher initial doses of 40-60 mg are recommended 3, 1

Maintenance Dosing

• After initial control of hyperthyroidism (typically within 4-12 weeks), the dose should be gradually reduced to the lowest effective maintenance dose 2
• Maintenance doses are often as low as 5-10 mg daily 1
• In many patients (66%), very low maintenance doses (≤5 mg/day) can be fully effective 2

Administration Options

• Carbimazole can be administered as a single daily dose or in divided doses 4
• A single daily dose (e.g., 30 mg at bedtime) has been shown to be as effective as divided doses (e.g., 10 mg every 8 hours) 4
• Single daily dosing may improve patient adherence 4

Dose-Related Side Effects

• The risk of side effects is dose-dependent, with significantly higher risk at:
  • Initial doses >30 mg for carbimazole (relative risk 1.6) 5
  • Initial doses >20 mg for thiamazole/methimazole (relative risk 2.3) 5
• Overall side effect rate is approximately 14.3%, with skin reactions being most common (5.6%) 5
• Agranulocytosis, the most serious side effect, occurs in approximately 0.14% of patients 5

Treatment Duration

• The mean treatment period is typically around 17 months 2
• Most side effects (97.1%) appear within the first year of treatment, with a median onset at one month 5
• Relapses occur most commonly (77%) within the first year after stopping treatment, but can occur up to 2-3 years later 2

Clinical Pearls

• Always use the lowest possible effective dose to minimize side effects 5
• Monitor thyroid function tests regularly to guide dose adjustments 2, 3
• Complete normalization of pituitary TSH reserve may take 6-12 months in most cases, even after clinical euthyroidism is achieved 2
• Long-term treatment with low maintenance doses appears to be well-tolerated 5