Carbimazole Treatment for Hyperthyroidism
Carbimazole should be initiated at 20-40 mg daily as a single dose, with 20 mg/day preferred for mild-to-moderate hyperthyroidism to minimize risk of iatrogenic hypothyroidism, while 40 mg/day is reserved for severe cases with baseline T4 >260 nmol/L. 1
Initial Dosing Strategy
Start with 20 mg daily for most patients with mild-to-moderate hyperthyroidism. This dose effectively controls thyroid hormone levels while carrying lower risk of drug-induced hypothyroidism compared to higher doses 1. The 20 mg dose achieves euthyroidism in approximately 4-5 weeks in most patients 2.
Use 40 mg daily for severe hyperthyroidism (baseline T4 >260 nmol/L), as lower doses are inadequate for controlling severe disease 1. The higher dose produces significantly lower T4 levels at 4 weeks (98 vs 158 nmol/L) and T3 levels (2.6 vs 4.3 nmol/L) compared to 20 mg dosing 1.
Administration Schedule
Give carbimazole as a single daily dose rather than divided doses. Single daily dosing (typically at bedtime) is equally effective as divided doses due to carbimazole's long intrathyroidal half-life, achieving euthyroidism in 3.8-4.6 weeks regardless of dosing schedule 2. This approach improves medication adherence 2.
Symptomatic Management
Add beta-blockers (propranolol or atenolol) for symptomatic relief while waiting for carbimazole to take effect 3. Beta-blockers control tachycardia, tremor, and other adrenergic symptoms during the initial weeks of treatment 3.
Treatment Duration and Monitoring
Continue carbimazole for 12-18 months using the titration method (adjusting to the lowest dose maintaining euthyroidism) 4. Treatment duration of 18 weeks (median) may be sufficient if stopped when T3 and T4 are in the low-normal range, typically 2-4 months after achieving clinical euthyroidism 5.
Monitor thyroid function every 2-4 weeks initially to adjust dosing and prevent iatrogenic hypothyroidism 3. Once stable, monitoring can be less frequent 4.
Dose Adjustment
Reduce or discontinue carbimazole if TSH becomes elevated or patient develops hypothyroid symptoms 1. The goal is maintaining euthyroidism with the minimum effective dose 4.
Important Caveats
Watch for agranulocytosis - if patients develop sore throat and fever, obtain complete blood count immediately and discontinue carbimazole 3. Other serious adverse effects include hepatitis, vasculitis, and thrombocytopenia 3.
Expect approximately 50% relapse rate after completing 12-18 months of therapy, at which point ablative therapy (radioiodine or surgery) should be offered 4. Shorter treatment courses (stopping when TSH response normalizes) may achieve 39% sustained remission rates 5.
In pregnancy, carbimazole (or its active metabolite methimazole) is the treatment of choice for hyperthyroidism, though propylthiouracil and methimazole show similar safety profiles 3. Use the lowest effective dose to maintain free T4 in the high-normal range 3.
Carbimazole is rarely needed for immune checkpoint inhibitor-induced thyrotoxicosis, which is typically transient thyroiditis managed with beta-blockers alone 3. Reserve carbimazole for persistent hyperthyroidism >6 weeks or confirmed Graves' disease with positive TSH receptor antibodies 3.