Influence of cigarette smoking on thyroid hormone levels.

1 Serum triiodothyronine (T3), thyroxine (T4) and thyrotropin (TSH) concentrations and urinary thiocyanate levels were examined in healthy smokers and non-smokers as an indicator of smoking behaviour. Smokers were subdivided into moderate and heavy. 2 Significant differences in urinary thiocyanate levels were apparent between all three groups. For heavy smokers, serum T3 concentrations were significantly above the values found in non-smokers. Increased serum T3 levels were not accompanied by a substantial change in serum T4 and TSH concentrations.