Smoking--a risk factor for hypothyroidism.

Smoking is associated with a spectrum of disorders. Recent reports have shown decreased serum concentrations of thyrotropin in euthyroid smokers, and there is an association between smoking and development of goiter (toxic and euthyroid). In a 12-year follow-up of a randomly selected sample of women we found a strong association between smoking at the time of initial screening and later development of hypothyroidism, the relative risk for a female smoker to develop hypothyroidism being 3.9 (95% confidence interval 1.6-9.1). There was, however, no association between smoking habits at the end of the follow-up and hypothyroidism. This indicates that several women who developed hypothyroidism may have done so in association with a change in smoking habits.