Thyroid Hormone Levels Associated with Active and Passive Cigarette Smoking

Background: Active and passive cigarette smoking are a risk factor among women of reproductive age-leading to reproductive health morbidity, including fetal and infant death and developmental problems with the newborn. However, the underlying physiological mechanisms for these ill-effects are not fully understood. Smoke exposure may affect various metabolic and biological processes, including hormone biosynthesis and secretion, interfere with thyroid hormone release, binding, transport, storage, and clearance, associated with adverse effects on the thyroid resulting in changes in circulating hormone concentrations. We measured and compared serum thyroid hormone and thyroid stimulating hormone (TSH) concentrations in active, passive, and nonsmokers and determined their association with cigarette tobacco smoke exposure. We use a comprehensive approach to assess the interrelationships between active and passive tobacco smoke exposure and thyroid hormone levels by employing innovative hormone analysis techniques. Methods: Serum was obtained from women (18-44 years of age). Thyroxine (T4), triiodothyronine (T3), and cotinine concentrations were quantified using isotope dilution high performance liquid chromatography tandem mass spectrometry, and TSH concentrations by chemiluminescence. Results: Serum concentrations of the various hormones of active smokers, passive smokers, and nonsmokers (nonexposed), respectively, were as follows. Median TSH concentrations were 1.02, 1.06, and 1.12 mIU/L (p < 0.001 for the comparison of each group with the other two groups), and mean TSH levels were 1.40 mIU/L, confidence interval (CI) 0.07-6.83 mIU/L; 1.30 mIU/L, CI 0.25-3.01 mIU/L; and 1.50 mIU/mL, CI 0.71-4.00 mIU/L. Median serum T4 concentrations were 7.6, 7.9, and 8.7 mu g/dL, and median serum T3 concentrations were 92.0, 84.0, and 102.0 ng/dL (p < 0.0001). Mean T3 levels were 99.1 ng/dL, CI 52.1-204.3 ng/dL; 87.6 ng/dL, CI 40.1-160.2 ng/dL; and 106.6 ng/dL, CI 46.4-175.0 ng/dL. Pair-wise comparisons of the three study groups indicate statistically significant differences in serum T4 (p < 0.01) and T3 (p < 0.001) means for the comparison of each group with the other two groups. Conclusions: This study is unique in examining the association of serum cotinine and thyroid hormone concentrations using liquid chromatography tandem mass spectrometry in women smokers, passive smokers, and nonsmokers. Active and passive exposure to cigarette tobacco smoke is associated with a mild inhibitory effect on the thyroid reflected in higher serum T4 and T3 in nonsmokers compared to smokers in this cohort of women.