Tobacco Smoking and Thyroid Function
Bjrn O. svold, MD; Trine Bjro, MD, PhD; Tom I. L. Nilsen, PhD; Lars J. Vatten, MD, PhD
Arch Intern Med. 2007;167(13):1428-1432. doi:10.1001/archinte.167.13.1428.
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.
The relation between tobacco smoking and thyroid function is not well understood. Most population-based studies show that smokers have lower levels of thyrotropin1- 4 and higher levels of thyroid hormones,2- 4 and smoking appears to increase the risk of Graves hyperthyroidism.5- 8 However, smoking cessation may reduce the risk of Graves hyperthyroidism,5 suggesting that hyperthyroid effects of smoking may be reversed in people who quit.
For hypothyroid disease, studies have shown either no association6,7 or an increased risk8,9 of hypothyroidism associated with smoking. Recent studies4,10 suggest that smokers are less likely to have elevated thyrotropin levels, and some studies10,11 have shown that smokers are less likely to have thyroid peroxidase antibodies, which may suggest that autoimmune thyroid disease could be less common in smokers. In a large, cross-sectional, population-based study from an iodine-sufficient area in Norway,12 we studied the association between smoking habits and thyroid function in people without previously known thyroid disease.
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