Associations of Urinary Cotinine-Verified Active and Passive Smoking with Thyroid Function: Analysis of Population-Based Nationally Representative Data

摘要

Background: The effects of active and passive smoking on thyroid function in the Korean population have not been determined. Furthermore, related research is based on self-reported smoking status, which may be inaccurate, especially among women. The present study aimed at evaluating the association between biochemically verified smoking status and thyroid function in a nationally representative Korean population. Methods: This population-based cross-sectional study included 3404 subjects without thyroid disease who were not taking thyroid medication. Smoking status was identified using self-reported data and urinary cotinine levels. Kruskal–Wallis and Jonckheere–Terpstra trend tests were performed to evaluate the association between smoking exposure and thyroid function. Multivariate logistic regression analysis was used to estimate the effect of smoking on subclinical hypothyroidism (SCH). Results: Biochemically verified active and passive smoking rates were 43.4% and 23.3% among men and 10.0% and 22.9% among women, respectively. Active smokers had significantly lower iodine levels than passive smokers and nonsmokers. Active smoking was associated with decreased serum thyrotropin (TSH) levels among both sexes, although only men exhibited a dose–response relationship between increasing smoking exposure and decreasing TSH levels. Passive smoking slightly decreased TSH levels, but the decrease was not statistically significant. The risk of SCH decreased with increasing smoking exposure in the multivariate-adjusted analysis ( p for trend = 0.027 among men and 0.042 among women). Conclusions: Active and passive smoking were associated with decreasing serum TSH levels and a lower risk of SCH in a Korean population. These associations might be related to lower urinary iodine levels in active smokers.