Carbimazole Dosing Schedule
For hyperthyroidism treatment, carbimazole should be administered as a single daily dose of 20-40 mg once daily, with 20 mg being preferred for mild-to-moderate disease and 40 mg reserved for severe hyperthyroidism, taken at any time of day (typically at bedtime for convenience). 1, 2
Standard Dosing Regimens
Initial Dose Selection
20 mg once daily is the recommended starting dose for patients with mild-to-moderate hyperthyroidism (baseline total T4 ≤260 nmol/L), as this dose effectively controls hyperthyroidism while minimizing the risk of iatrogenic hypothyroidism 1
40 mg once daily should be used for patients with severe hyperthyroidism (baseline total T4 >260 nmol/L), as lower doses are less effective at controlling severe disease 1
30 mg once daily represents a middle-ground option that has been validated in multiple studies and achieves euthyroidism in 1-3 months for most patients 2, 3
Timing and Administration
Single daily dosing is as effective as divided doses (three times daily) because carbimazole has a long intrathyroidal half-life despite a short plasma half-life 3, 4
Bedtime administration is commonly recommended for convenience and improved compliance, though the drug can be taken at any time of day 3
Propranolol 20 mg three times daily should be co-administered for the first 3-4 weeks to control adrenergic symptoms 2, 4
Monitoring and Dose Adjustment
Thyroid function tests (total T4, total T3, TSH) should be checked at baseline, then at 4 weeks and 10 weeks after starting therapy 1
Patients on 20 mg daily typically achieve euthyroidism in 4-6 weeks, while those on 40 mg daily achieve biochemical control faster (by 4 weeks) but have higher risk of iatrogenic hypothyroidism 1
The 40 mg dose produces significantly lower thyroid hormone levels at 4 weeks (free T4: 19.4 vs 35.2 pmol/L; free T3: 8.3 vs 13.7 pmol/L) compared to 20 mg, but clinical symptoms improve similarly by 6-12 weeks 1
Duration of Therapy
Treatment should continue until TSH response to TRH normalizes, typically 9-41 weeks (median 18 weeks) 2
Stopping carbimazole when T3 and T4 are in the low-normal range (usually 2-4 months after clinical euthyroidism) may improve remission rates 2
Important Caveats
Drug Resistance
Rarely, patients may not respond to standard or even supratherapeutic doses of carbimazole (up to 60 mg daily) 5
If no biochemical improvement occurs after 4 months of compliant therapy, consider carbimazole resistance and switch to propylthiouracil 150 mg three times daily or proceed to definitive therapy (radioiodine ablation or thyroidectomy) 5
Always verify medication compliance through supervised administration before diagnosing drug resistance 5
Predictors of Response
Patients with large goiters and elevated alkaline phosphatase take longer to respond to therapy 2
Severe hyperthyroidism (T4 >260 nmol/L) requires the higher 40 mg dose for adequate control 1
Pregnancy Considerations
- In pregnant women with hyperthyroidism, thioamides (propylthiouracil or methimazole/carbimazole) should be used at the lowest possible dose to maintain free T4 in the high-normal range, with monitoring every 2-4 weeks 6