Tobacco Smoking and Thyroid Function

Tobacco Smoking and Thyroid Function



A Population-Based Study



Bjørn O. Åsvold, MD; Trine Bjøro, MD, PhD; Tom I. L. Nilsen, PhD; Lars J. Vatten, MD, PhD



Arch Intern Med. 2007;167:1428-1432.



Background The association between tobacco smoking and thyroid function is incompletely understood.



Methods In a cross-sectional, population-based study conducted
between August 15, 1995, and June 18, 1997, of 20 479 women and 10 355
men without previously known thyroid disease, we calculated the
geometric mean serum concentration of thyrotropin and the prevalence of
hypothyroidism and hyperthyroidism among current, former, and never
smokers.



Results Among women, the mean thyrotropin level was lower in
current (1.33 mIU/L; 95% confidence interval [CI], 1.29-1.36 mIU/L) and
former smokers (1.61 mIU/L; 95% CI, 1.56-1.65 mIU/L) compared with
never smokers (1.66 mIU/L; 95% CI, 1.63-1.70 mIU/L). Similarly, among
men, the mean thyrotropin level was lower in current (1.40 mIU/L; 95%
CI, 1.36-1.44 mIU/L) and former smokers (1.61 mIU/L; 95% CI, 1.57-1.66
mIU/L) compared with never smokers (1.70 mIU/L; 95% CI, 1.66-1.75
mIU/L). In former smokers, thyrotropin levels increased gradually with
time since smoking cessation (P for trend < .001). Among current
smokers, moderate daily smoking was associated with higher thyrotropin
levels than heavier smoking. In women, the prevalence of overt
hypothyroidism was lower in current smokers compared with never smokers
(odds ratio, 0.60; 95% CI, 0.38-0.95), whereas the prevalence of overt
hyperthyroidism was higher among current smokers (odds ratio, 2.37; 95%
CI, 1.34-4.20). The associations related to subclinical thyroid
dysfunction were similar to those for overt thyroid disease.



Conclusions These findings indicate that smoking is negatively
associated with hypothyroidism but positively associated with
hyperthyroidism. The associations with smoking cessation suggest that
smoking may have reversible effects on thyroid function. Notably, we
report for the first time, to our knowledge, a lower prevalence of
overt hypothyroidism among current smokers.