Carbimazole Dosing for Hyperthyroidism
No, 5mg is not the default starting dose of carbimazole for treating hyperthyroidism—the standard initial dose is 20-40mg daily, with 30mg daily being most commonly used. 1, 2, 3
Initial Dosing Strategy
For mild to moderate hyperthyroidism, start with 20mg daily as a single dose. This provides effective control while minimizing the risk of iatrogenic hypothyroidism. 4 Patients with baseline total T4 >260 nmol/L (severe hyperthyroidism) require higher initial doses of 40mg daily to achieve adequate biochemical control. 4
For typical cases, 30mg daily given as a single dose at bedtime is highly effective. This achieves euthyroidism within 1-3 months in most patients and offers superior convenience compared to divided dosing. 2, 3 The single daily dose approach is equally effective as divided doses (10mg three times daily) based on the drug's long intrathyroidal half-life. 2, 5
Dose Titration and Maintenance
After achieving biochemical control (typically 4-12 weeks), gradually reduce to the lowest effective maintenance dose. 6 Remarkably, 66% of patients can be maintained on very low doses (≤5mg daily) with comparable remission rates of 54%. 6 The maintenance dose does not affect remission rates, so aggressive dose reduction is appropriate once control is established. 6
Continue treatment for a median of 17-18 weeks (range 9-41 weeks) until TSH response to TRH normalizes. 6, 3 Most patients achieve normal TSH reserve after 6-12 months of treatment. 6
Pregnancy-Specific Dosing
In pregnant women beyond the first trimester, use the lowest possible dose to maintain free T4 in the high-normal range, with monitoring every 2-4 weeks. 1 Propylthiouracil is preferred during the first trimester due to possible teratogenicity of methimazole/carbimazole. 7, 1
Symptomatic Management
Add beta-blockers (atenolol 25-50mg daily or propranolol) for the first 3 weeks or until thyroid hormones normalize. 1 Continue beta-blocker therapy to maintain heart rate <90 bpm if blood pressure tolerates. 1
Critical Safety Monitoring
If a patient develops sore throat and fever, immediately obtain a complete blood count and discontinue carbimazole due to agranulocytosis risk. 1 Also monitor for hepatitis, vasculitis, and thrombocytopenia. 1
Common Pitfalls to Avoid
- Don't start at 5mg: This dose is only appropriate for maintenance therapy after initial control, not as initial treatment
- Don't continue high doses unnecessarily: Reduce to ≤5mg daily when possible to minimize side effects while maintaining efficacy 6
- Don't stop monitoring after achieving euthyroidism: TSH normalization lags behind clinical improvement by 6-12 months 6
- Don't assume remission is permanent: 77% of relapses occur within the first year after stopping treatment, but relapses can occur 2-3 years later, requiring extended follow-up 6