J Korean Endocr Soc.  2008 Apr;23(2):123-128. 10.3803/jkes.2008.23.2.123.

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

• Kim BH
• Kim WB
• Kim TY
• Kim HK
• Jeon SH
• Lee CW
• Shong YK

• KMID: 2063563
• DOI: http://doi.org/10.3803/jkes.2008.23.2.123

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.

Keyword

hyperthyroidism; hypothyroidism; prevalence; smoking; thyrotropin

MeSH Terms

Adult
Cross-Sectional Studies
Estrogens
Female
Health Promotion
Humans
Hyperthyroidism
Hypothyroidism
Korea
Male
Prevalence
Smoke
Smoking
Thyroid Diseases
Thyroid Gland
Thyrotropin
Tobacco Products
Estrogens
Smoke
Thyrotropin

Figure

• 

  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.

• 

  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.

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