Effects of cigarette smoking on thyroid function and the prevalence of goitre, thyrotoxicosis and autoimmune thyroiditis.

The effect of smoking habits on thyroid function and the prevalence of thyroid diseases was studied in two birth cohorts (1928 and 1941) of men and women. A total of 1555 individuals (836 men and 719 women) were smokers. 1048 individuals (604 men and 444 women) were ex-smokers and 1497 individuals (560 men and 937 women) were non-smokers. The serum thyrotropin (TSH) concentration was significantly lower in smokers than in ex-smokers and non-smokers (P = 0.05 and P = 0.001, respectively). Smoking habits did not affect the serum triiodothyronine (T3) concentrations. The prevalence of non-toxic goitre and toxic diffuse goitre was significantly higher in smoking women than in non-smoking women (P = 0.005 and P = 0.04, respectively). Thiocyanate may be the goitrogenic factor responsible for the increased prevalence of non-toxic goitre. The cause of the increased prevalence of toxic diffuse goitre is less obvious. Hypothetically, an increase in sympathetic activity in smokers may promote the development of thyrotoxicosis in predisposed individuals, but immunological disturbances caused by smoking are another possibility.