Recent studies have shown a possible relationship between cigarette smoking and endocrine function. Since changes in thyroid hormone concentrations can have a profound effect on overall metabolism as well as influencing both androgenic and estrogenic steroid activity, we investigated the relationship between cigarette smoking and thyroid function. Volunteers were divided into four groups according to self-reported and biochemical data, including blood carboxyhemoglobin, plasma cotinine, and thiocyanate, and were described as nonsmokers, light, moderate, and heavy smokers. Substantial decreases were found in serum thyroxine (T4) and triiodothyronine (T3) concentrations in heavy smokers compared with nonsmokers. Reduced serum thyroid hormone levels were not accompanied by a substantial change in serum thyrotropin concentrations. The free T4 index, free T3 index, and T4-T3 ratio were not substantially different for either group.