ABSTRACT. Serum concentrations of T4, T3, rT3 and TSH were determined before and after cessation of chronic smoking in healthy subjects. Their body weights were measured at the same intervals. Abstention from smoking was monitored through determination of CO content in blood. Cessation of smoking was accompanied by small reductions in serum T4 and rT3 and by a small rise in serum TSH. Serum T3 was not significantly altered. A minor rise in body weight was noticed. It is suggested that smoking promotes a modestly increased secretion of thyroid hormone, which is readjusted to normal following cessation of smoking. The assumed smoking‐induced increase in thyroid hormone secretion is apparently not mediated by enhanced TSH secretion but by some other factor, e.g. the sympathetic nervous system. The hormonal changes might contribute to the increase in body weight frequently seen after cessation of smoking.