What's the initial dose of carbimazole for hyperthyroidism due to Graves' disease?

Restarting Carbimazole for Severe Graves' Hyperthyroidism

Start carbimazole at 15-20 mg once daily, not the standard 30-40 mg dose, given the markedly elevated free T4 of 60 pmol/L and suppressed TSH, as patients with severe biochemical hyperthyroidism may be hypersensitive to antithyroid drugs and can develop rapid overcorrection. 1

Initial Dose Selection

• Begin with 15-20 mg carbimazole once daily rather than the conventional 30-40 mg starting dose 2, 3
• The severely elevated FT4 (60 pmol/L, approximately 3-4 times upper limit of normal) indicates marked thyroid hormone excess that paradoxically predicts rapid response to antithyroid drugs 1, 3
• Patients with very high initial FT4 levels (>40 pmol/L) can experience unexpectedly rapid falls in thyroid hormone levels within 3-9 weeks on conventional or even low doses of carbimazole 1

Critical Monitoring Protocol

• Recheck thyroid function tests (TSH, FT4, FT3) in 2-3 weeks after initiating therapy, not the standard 4-6 weeks 1
• Continue monitoring every 2-3 weeks until FT4 normalizes, as this patient is at high risk for rapid overcorrection 1
• Once FT4 reaches low-normal range, switch to 4-6 week monitoring intervals 2

Dose Titration Strategy

• Reduce carbimazole by 5 mg increments if FT4 falls to low-normal or below normal range, even if TSH remains suppressed 1
• Target FT4 in the low-normal range (12-18 pmol/L) before considering dose reduction 2
• Be prepared for a "yo-yo" pattern where FT4 fluctuates between low and normal levels, requiring fine dose adjustments of 2.5-5 mg 1
• Maintenance doses may be as low as 2.5-5 mg daily in hypersensitive patients 1

Adjunctive Symptomatic Management

• Add propranolol 40 mg three times daily (or atenolol 25-50 mg once daily) for the first 3-4 weeks to control adrenergic symptoms including tremor, palpitations, and anxiety 2, 4
• Beta-blockers provide rapid symptomatic relief while waiting for carbimazole to take effect 4
• Taper beta-blocker once clinical euthyroidism is achieved, typically within 4-8 weeks 2

Common Pitfalls to Avoid

• Never start with 40 mg carbimazole in patients with FT4 >50 pmol/L, as this dramatically increases risk of iatrogenic hypothyroidism within weeks 1, 3
• Do not wait 4-6 weeks for first follow-up in severely hyperthyroid patients—overcorrection can occur rapidly and cause symptomatic hypothyroidism 1
• Avoid relying solely on TSH for dose adjustment in the first 2-3 months, as TSH remains suppressed for weeks to months after FT4 normalizes 1
• Do not assume standard dosing applies—younger patients (<30 years) may require higher doses (30-40 mg), while those with very high FT4 require lower starting doses 3, 1

Expected Response Timeline

• Clinical improvement (reduced tremor, palpitations, heat intolerance) typically occurs within 1-3 weeks 2
• Biochemical euthyroidism (normal FT4) usually achieved within 1-3 months on appropriate dosing 2
• TSH normalization lags behind FT4 normalization by 2-4 months 2

Special Considerations for This Patient

• The combination of severely suppressed TSH (0.015 mIU/L) and markedly elevated FT4 (60 pmol/L) suggests severe, active Graves' disease 3
• If thyroid gland is small on examination or ultrasound, this further increases likelihood of rapid response and need for lower starting dose 1
• Monitor for adverse effects including rash (most common), agranulocytosis (rare but serious), and hepatotoxicity, which occur in approximately 11.5% of patients 3

When to Seek Specialist Input

• If FT4 fails to decrease by at least 30-40% within 4-6 weeks, consider endocrinology referral for possible radioactive iodine therapy 5, 3
• If patient develops severe adverse effects (rash, fever, sore throat, jaundice), stop carbimazole immediately and refer urgently 3
• If "yo-yo" pattern of thyroid function persists despite dose adjustments, specialist input may help optimize management 1