Treatment of Hyperthyroidism with Neomercazole (Carbimazole)
The recommended initial treatment for hyperthyroidism using Neomercazole (carbimazole) is 20-30 mg daily for mild to moderate cases, with higher doses of 40 mg daily reserved for severe hyperthyroidism, followed by maintenance therapy of 5-15 mg daily once euthyroidism is achieved. 1, 2
Initial Dosing Strategy
Dosing Based on Disease Severity
Mild to moderate hyperthyroidism:
Severe hyperthyroidism (TT4 > 260 nmol/L):
Administration Schedule
- Carbimazole can be administered as a single daily dose rather than divided doses
- Single daily dosing has comparable efficacy to divided dosing and improves compliance 3, 4
- A single 30 mg daily dose has been shown to effectively control hyperthyroidism 4
Monitoring and Dose Adjustment
Initial Monitoring
- Check thyroid function tests (TSH, Free T4) at 4 weeks after starting treatment 2
- Patients on 40 mg/day show lower T4 and T3 levels at 4 weeks compared to those on 20 mg/day 2
Dose Titration
- Once clinical and biochemical improvement occurs (usually within 4-6 weeks):
- Reduce to a maintenance dose of 5-15 mg daily
- Adjust dose to maintain TSH and Free T4 within normal range 1
- Goal is to maintain Free T4 or Free T4 Index in the high-normal range using the lowest possible thioamide dosage 1
Long-term Monitoring
- Regular monitoring of thyroid function every 2-4 weeks during initial treatment 1
- Once stable, monitoring can be less frequent (every 2-3 months)
- Treatment typically continues for 12-18 months to increase chances of remission 4
Adjunctive Treatments
Beta Blockers
- Until carbimazole reduces thyroid hormone levels, a beta blocker (e.g., propranolol) can be used to control symptoms 1, 5
- Propranolol 60-80 mg orally every 4-6 hours is recommended for symptomatic relief 5
- Non-dihydropyridine calcium channel antagonists (diltiazem or verapamil) are alternatives when beta-blockers are contraindicated 5
Combination Therapy
- Adding thyroxine (T4) to carbimazole treatment does not prevent recurrence of hyperthyroidism after withdrawal of antithyroid drugs 6
Special Considerations
Pregnancy
- Carbimazole (Neomercazole) is contraindicated in the first trimester of pregnancy due to risk of congenital anomalies 1, 5
- Propylthiouracil is preferred during the first trimester 1, 5
- Consider switching to carbimazole for second and third trimesters due to propylthiouracil's hepatotoxicity risk 5
Elderly Patients
- Lower starting doses (10-20 mg daily) are recommended
- Elderly patients are more susceptible to adverse effects and iatrogenic hypothyroidism 5
Monitoring for Adverse Effects
Common Side Effects
- Gastrointestinal disturbances
- Skin rashes
- Pruritus
- Hair loss
Serious Adverse Effects
Agranulocytosis:
- Rare but serious side effect (0.1-0.5% of patients)
- Usually presents with sore throat and fever
- If these symptoms develop, obtain a complete blood count and discontinue carbimazole 1
Hepatotoxicity:
- Monitor liver function tests periodically
- Discontinue if significant elevation in liver enzymes occurs
Thyroid dysfunction:
- Hypothyroidism can occur with excessive dosing
- Regular monitoring of thyroid function is essential 5
Treatment Duration and Outcomes
- Typical treatment duration is 12-18 months
- Approximately 40-50% of patients achieve remission after this period 4
- Consider definitive treatment (radioactive iodine or surgery) for patients who relapse after a course of carbimazole
Practical Tips
- Patient education about symptoms of agranulocytosis is essential
- Advise patients to seek immediate medical attention if they develop sore throat, fever, or mouth ulcers
- Lower doses (5-10 mg daily) have been shown to effectively reduce intrathyroidal iodide binding and may be sufficient for maintenance therapy 7
- Untreated hyperthyroidism increases cardiovascular risk, particularly atrial fibrillation and cardiac dysfunction 5