What is the appropriate initial carbimazole dose for a 43-year-old woman with overt hyperthyroidism (free T3 13.7 pg/mL, free T4 3.7 ng/dL, TSH < 0.005 µIU/mL)?

Carbimazole Dosing for Overt Hyperthyroidism

Immediate Treatment Recommendation

Start carbimazole 40 mg once daily immediately, along with propranolol 20-40 mg three to four times daily to control tachycardia and prevent thyroid storm. 1

Your patient has severe overt hyperthyroidism with markedly suppressed TSH (<0.005 µIU/mL), elevated free T3 (13.7 pg/mL), and elevated free T4 (3.7 ng/dL), requiring aggressive initial therapy.

Rationale for 40 mg Daily Dosing

For severe hyperthyroidism with baseline T4 levels substantially elevated (as in this case), 40 mg/day carbimazole is more effective than 20 mg/day at achieving rapid biochemical control. 2

• Patients receiving 40 mg/day carbimazole achieve significantly lower total T4 (98 vs 158 nmol/L, P<0.001) and free T3 (8.3 vs 13.7 pmol/L, P<0.01) at 4 weeks compared to those receiving 20 mg/day 2
• The 20 mg/day dose is less effective at controlling hyperthyroidism in patients with severe disease (baseline T4 >260 nmol/L) 2
• Higher initial doses are required for severe hyperthyroidism to prevent progression to thyroid storm 2

Essential Concurrent Beta-Blocker Therapy

Propranolol 20-40 mg orally 3-4 times daily (or atenolol 25-50 mg once daily) must be started immediately to control tachycardia and prevent cardiovascular complications. 1

• Titrate the beta-blocker dose to achieve heart rate <90 bpm if blood pressure tolerates 1
• Beta-blockers provide symptomatic relief while waiting for carbimazole to take effect (typically 1-4 weeks) 1

Critical Diagnostic Testing Before Finalizing Treatment

Order TSH receptor antibodies (TRAb), thyroid peroxidase antibodies (TPO), total T3 or free T3, and thyroid ultrasound to differentiate Graves' disease from thyroiditis. 1

This distinction is crucial because:

• Thyroiditis is self-limiting and requires only symptomatic treatment with beta-blockers, not long-term antithyroid drugs 1
• Graves' disease requires 12-18 months of antithyroid medication or definitive therapy with radioactive iodine 1
• Positive TSH receptor antibodies and high radioiodine uptake confirm Graves' disease 1
• Low/absent radioiodine uptake indicates thyroiditis 1

Monitoring Protocol

Recheck TSH, free T4, and free T3 every 2-3 weeks initially to monitor response and adjust dosing. 1

• Once biochemically euthyroid (typically 4-8 weeks), reduce carbimazole to a maintenance dose of 5-15 mg daily 2, 3
• The risk of iatrogenic hypothyroidism is lower with 20 mg/day than 40 mg/day, so dose reduction is essential once control is achieved 2
• Continue monitoring every 4-6 weeks during maintenance therapy 3

Expected Clinical Course

Clinical euthyroidism typically occurs within 1-3 months, with normalization of serum T4 levels. 3

• Patients with large goiters and elevated alkaline phosphatase may take longer to respond 3
• If thyroiditis is confirmed, expect transition from thyrotoxic phase (lasting 1-4 weeks) to hypothyroid phase within 1-2 months 1
• For Graves' disease, continue carbimazole for 12-18 months before attempting withdrawal 3, 4

Common Pitfalls to Avoid

Do not start with 20 mg/day in severe hyperthyroidism—this dose is inadequate for patients with markedly elevated thyroid hormones and increases the risk of persistent hyperthyroidism 2

Do not delay beta-blocker therapy—cardiovascular complications including atrial fibrillation and thyroid storm can develop rapidly in untreated severe hyperthyroidism 1

Do not assume Graves' disease without confirmatory testing—thyroiditis requires completely different management and stopping carbimazole once the thyrotoxic phase resolves 1

Refer to endocrinology within 1 week for definitive diagnosis and long-term management planning 1