Ten patients with severe dyspnoea and chronic airflow obstruction entered a randomised double-blind crossover trial comparing the effect of carbimazole 80 mg daily for two months with that of placebo. Assessment of thyroid function, lung function, and exercise tolerance was performed monthly. The mean free thyroxine index after two months of carbimazole was significantly lower at 64.1 (+/- 10.5, SEM) than the 89.1 (+/- 3.8) while on placebo. Serum tri-iodothyronine was reduced and thyroid stimulating hormone raised while on the active drug. There was no significant difference in the 12-minute walking distance (TMD), the rating of perceived exertion during the TMD, the oxygen cost score, the dyspnoea grade, the resting arterialised capillary blood gas tensions or the resting minute ventilation. During a progressive exercise test to exhaustion on a cycle ergometer, there was no significant difference in the minute ventilation, heart rate, blood gas tensions at exhaustion, or the total work done. There were no symptoms or signs of hypothyroidism. Lung function (FEV1, FVC, TLC, KCO) was unchanged. Thus a 28% reduction in the free thyroxine index produced no symptomatic or objective benefit in exercise tolerance in patients with severe airflow obstruction. These results provide no support for the use of carbimazole in chronic airflow obstruction.
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