A comparison of 20 or 40 mg per day of carbimazole in the initial treatment of hyperthyroidism

The optimal dosage regimen for carbimazole (CBZ) in the treatment of hyperthyroidism remains uncertain, despite clinical use of the drug for approximately fifty years. We have compared the early clinical and biochemical responses to 20 or 40 mg/day of CBZ given as initial treatment for hyperthyroidism.

Prospective open multicentre trial.

Sixty-three patients presenting with hyperthyroidism.

Serum total and free thyroid hormones, serum TSH and SHBG were measured at baseline and at 4 and 10 weeks after start of therapy. Weight, pulse and a symptom questionnaire were also monitored at 6 and 12 weeks.

Patients randomized to a starting dose of 40 mg/day CBZ had lower total (98 +/- 10 vs 158 +/- 11 nmol/l, P < 0.001) and free T4 (19.4 +/- 2.6 vs 35.2 +/- 3.7 pmol/l, P < 0.001) and total (2.6 +/- 0.3 vs 4.3 +/- 0.4 nmol/l, P < 0.001) and free T3 (8.3 +/- 1.0 vs 13.7 +/- 1.2 pmol/l, P < 0.01) at 4 weeks than those receiving 20 mg/day. Clinical responses at 6 and 12 weeks (weight, pulse, symptom score) and SHBG concentrations were similar. Drug-related hypothyroidism was less likely to occur at 4 and 10 weeks in those patient who initially received 20 mg CBZ/day, but this dose was less effective at controlling hyperthyroidism in those with more severe hyperthyroidism with baseline TT4 > 260 nmol/l.

In treating hyperthyroidism, 20 mg/day carbimazole is effective, convenient and has a lower risk than 40 mg/day of iatrogenic hypothyroidism in patients with mild or moderate hyperthyroidism. Higher doses are required for those with severe hyperthyroidism.