Cigarette smoking and risk of clinically overt thyroid disease - A population-based twin case-control study

Background: The effects of cigarette smoking on the thyroid gland have been studied for years. However, the effect of smoking on thyroid function and size is still controversial. Objective: To determine the impact of cigarette smoking on the development of clinically overt thyroid disease. Methods: Matched case-control study of 132 same-sex twin pairs (264 individ uals) discordant for clinically overt thyroid disease, ascertained from a p opulation-based nationwide twin register. Information on thyroid disease an d smoking habits was gathered by questionnaire, and the patients' endocrino logist or general practitioner verified the diagnosis. Results: Overall, smoking was associated with an increased risk of developi ng clinically overt thyroid disease (odds ratio,3.0; 95% confidence interva l, 1.4-6.6; P =.003). This association remained statistically significant i n monozygotic and dizygotic disease-discordant pairs. The effect of smoking was more pronounced in,monozygotic vs dizygotic pairs (odds ratio, 5.0 vs 2.5; P=.04 for both). Essentially similar results were obtained after subdi viding the twin pairs into groups discordant for clinically overt autoimmun e (49 pairs) and nonautoimmune (83 pairs) thyroid disease. Among twin pairs concordant for smoking, probands with clinically overt autoimmune thyroid disease smoked significantly more than did their healthy co-twins (17 pairs ; P=.03), whereas no difference was found between probands with nonautoimmu ne thyroid disease and their healthy co-twins (34 pairs; P=.20). Conclusions: Smoking is associated with an increased risk of developing cli nically overt thyroid disease. Furthermore, our data suggest that cumulativ e cigarette consumption is a risk factor, most pronounced in autoimmune thy roid disease.