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Kim, Kim, Kim, Kim, Jeon, Lee, and Shong: Association between Cigarette Smoking and Thyroid Function in Adults without Previous History of Thyroid Disease
Original Article
Journal of Korean Endocrine Society

Journal of Korean Endocrine Society 2008; 23(2): 123-128.

Published online: 20 April 2008

DOI: https://doi.org/10.3803/jkes.2008.23.2.123

Association between Cigarette Smoking and Thyroid Function in Adults without Previous History of Thyroid Disease

Bo Hyun Kim, Won Bae Kim1, Tae Yong Kim1, Hong Kyu Kim2, Seong Hoon Jeon2, Chang Won Lee, Young Kee Shong, 1

Department of Internal Medicine, Busan St. Mary's Medical Center, Korea.

1Department of Internal Medicine, Asan Medical Center, University of Ulsan College of Medicine, Korea.

2Health Promotion Center, Asan Medical Center, University of Ulsan College of Medicine, Korea.

Received 13 November 2007       Accepted 3 February 2008

Copyright © 2008 Korean Endocrine Society

Abstract

Background

Cigarette smoking has a significant impact on thyroid function. However, the association between smoking and thyroid function is incompletely understood.

Methods

We conducted a cross-sectional study that included 90,970 adults (age range: 20 to 79) who had visited the health promotion center at Asan Medical Center between January 1, 2001, and December 31, 2003. Those subjects with previous known thyroid disease, a history of thyroid operation, a history of thyroid function altering medication (herb, estrogen or digestive) or a family history of thyroid disease were excluded. Finally, 47,577 subjects (males: 30,726, females: 16,851) were included in this study. We calculated the age-adjusted geometric mean of the serum TSH and the age-adjusted mean of the serum free T4 among the current, former and never smokers. We also analyzed the age-adjusted prevalence of hypothyroidism and hyperthyroidism among each group.

Results

Among men, the geometric mean TSH level was significantly low in the current (1.40 mIU/L, 95% confidence interval [CI]: 1.38-1.41) and former smokers (1.59 mIU/L, 95% CI: 1.57-1.61) compared with the never smokers (1.65 mIU/L, 95% CI: 1.63-1.68). The mean free T4 level was high in the current smokers (1.236 ng/dL, 95% CI: 1.234-1.239) compared with the never smokers (1.234 ng/dL, 95% CI: 1.230-1.238). Similarly, among women, the geometric mean TSH level was low in the current smokers (1.75 mIU/L, 95% CI: 1.67-1.87) compared with the never smokers (1.85 mIU/L, 95% CI: 1.83-1.87). The mean free T4 level was high in the current smokers (1.149 ng/dL, 95% CI: 1.139-1.159) compared with the never smokers (1.138 ng/dL, 95% CI: 1.135-1.140). Among the current male smokers, heavy daily smoking (over 2 packs per day) was more associated with low TSH levels than moderate smoking (less than 1 pack per day). In men, the prevalence of subclinical hypothyroidism was low in the current smokers compared with the never smokers (odds ratio: 0.53, 95% CI: 0.43-0.66).

Conclusion

We found that current smokers had lower TSH levels and higher free T4 levels than never smokers in both men and women and smoking was associated with a low prevalence of subclinical hypothyroidism in men, which may be of importance when evaluating subjects with subclinical hypothyroidism in Korea.

Keywords: hyperthyroidism, hypothyroidism, prevalence, smoking, thyrotropin

Figures and Tables

Fig. 1
Thyroid function according to smoking status in men; (A) age-adjusted geometric mean of TSH, (B) age-adjusted mean of free T4. Data was mean with standard deviation. *P < 0.001. †P < 0.01. ‡P < 0.05.
jkes-23-123-g001
Fig. 2
Thyroid function according to smoking status in women; (A) age-adjusted geometric mean of TSH, (B) age-adjusted mean of free T4. Data was mean with standard deviation. †P < 0.01. ‡P < 0.05.
jkes-23-123-g002
Table 1
Age-adjusted odd ratios and 95 percent confidence intervals for smoking status associated with thyroid dysfunction in men and women
jkes-23-123-i001

*defined as a TSH level greater than 5.0 mIU/L combined with a free T4 level of 0.8 ng/dL or more.

†defined as a TSH level greater than 5.0 mIU/L combined with a free T4 level less than 0.8 ng/dL.

‡defined as a TSH level less than 0.4 mIU/L combined with a free T4 level of 1.9 ng/dL or less.

§defined as a TSH level less than 0.4 mIU/L combined with a free T4 level greater than 1.9 ng/dL

CI, confidence interval; OR, odds ratio.

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