What is the recommended dosing regimen for Neomercazole (Carbimazole) in the treatment of hyperthyroidism?

Neomercazole (Carbimazole) Prescribing Guide

Start with carbimazole 30 mg once daily as a single morning dose for most patients with hyperthyroidism, adjusting to 20 mg daily if hyperthyroidism is mild to moderate, and titrate based on free T4 levels every 2-4 weeks to maintain thyroid hormones in the high-normal range using the lowest effective dose. 1, 2, 3

Initial Dosing Strategy

Standard starting dose:

• 30 mg once daily for moderate to severe hyperthyroidism 2, 4
• 20 mg once daily for mild to moderate hyperthyroidism (baseline total T4 <260 nmol/L) to reduce risk of iatrogenic hypothyroidism 3
• 40 mg daily may be required for severe hyperthyroidism (baseline total T4 >260 nmol/L) 3

Timing of administration:

• Give as a single daily dose, preferably in the morning 2, 4
• Single daily dosing is as effective as divided doses due to carbimazole's long intrathyroidal half-life 4

Pregnancy Considerations

Critical safety note: Avoid carbimazole in the first trimester of pregnancy—use propylthiouracil instead during this period 1

For pregnant women beyond first trimester:

• Use the lowest possible dose to maintain free T4 in the high-normal range 5
• Monitor free T4 or free thyroxine index every 2-4 weeks 5
• Breastfeeding is safe with thioamide therapy 5

Symptomatic Management

Add beta-blocker for symptom control:

• Atenolol 25-50 mg daily to maintain heart rate <90 bpm (if blood pressure allows) 1
• Alternative: propranolol or metoprolol 1
• If beta-blockers contraindicated: diltiazem or verapamil 1
• Continue beta-blocker for first 3 weeks or until thyroid hormones normalize 5

Monitoring Protocol

Frequency of monitoring:

• Check free T4 every 2 weeks initially for symptomatic patients 1
• Once stable, check every 2-4 weeks 5
• Clinical euthyroidism typically achieved in 1-3 months 2
• Biochemical euthyroidism (normal T4) typically achieved in 3.8-4.6 weeks 4

Target thyroid function:

• Maintain free T4 in the high-normal range 5
• Use the lowest possible dose to achieve this target 5

Duration of Treatment

Treatment course:

• Continue carbimazole until TSH response to TRH normalizes 2
• Typical treatment duration: median 18 weeks (range 9-41 weeks) 2
• Consider stopping when T3 and T4 are in the low-normal range, usually 2-4 months after clinical euthyroidism 2
• Approximately 39% of patients achieve prolonged remission (>1 year) with this approach 2

Critical Safety Monitoring

Agranulocytosis warning:

• Immediately obtain CBC and discontinue carbimazole if patient develops sore throat and fever 5, 1
• This is the most serious side effect requiring urgent action 5

Other side effects to monitor:

• Hepatitis 5
• Vasculitis 5
• Thrombocytopenia 5

Special Populations

Patients with large goiters or elevated alkaline phosphatase:

• Expect longer time to achieve euthyroidism 2
• May require extended treatment duration 2

Contraindications:

• Untreated hyperthyroidism in patients requiring phentermine-containing medications (due to arrhythmia and seizure risk) 5

Dose Adjustment Algorithm

Low-dose efficacy:

• Doses as low as 5-10 mg daily can markedly reduce intrathyroidal iodide binding 6
• 10 mg daily may be effective as maintenance in many patients once initial control achieved 6
• Titrate down to minimum dose that maintains biochemical control 5