This retrospective study examined the relationship between smoking history and thyroid function in 387 women patients with Hashimoto's thyroiditis (mean age +/- SD = 50.5 +/- 12.7 yr). The same analysis was done in 238 randomly chosen women patients with nodular goiters (mean age = 45.3 +/- 14 yr) and 166 control women (mean age = 47.7 +/- 14.2 yr). In patients with Hashimoto's thyroiditis, there were 256 non smokers, 110 smokers, and 21 smokers. Among the 110 smoking patients with Hashimoto's thyroiditis, 76.4% were hypothyroid, whereas the prevalence of hypothyroidism was 34.8% among the 256 non smokers. Among the 21 ex-smokers with Hashimoto's thyroiditis, the majority of patients (61.9%) were hypothyroid, suggesting that cessation of smoking does not appear to reverse hypothyroidism. The percentages of smokers in the hypothyroid group, the subclinical hypothyroid group, and the euthyroid group were 45.2%, 18%, and 11.3%, respectively, in patients with Hashimoto's thyroiditis. The greatest serum levels of thiocyanate (an antithyroid substance generated by smoking) were found in those who both smoked and had hypothyroidism. Thus, an increase in serum thiocyanate concentration from smoking may contribute to the development of hypothyroidism in patients with Hashimoto's thyroiditis. Smoking related hypothyroidism was not seen in patients with nodular goiters. Our results suggest that smoking may increase the risk of hypothyroidism in patients with Hashimoto's thyroiditis.