Serum thyroid-stimulating hormone levels and smoking status: Data from the Korean National Health and Nutrition Examination Survey VI.

OBJECTIVE

We aimed to evaluate differences in serum thyroid-stimulating hormone (TSH) levels according to smoking status and urine iodine concentration (UIC) in a healthy Korean population using data from the Sixth Korean National Health and Nutrition Examination Survey (KNHANES VI).

STUDY DESIGN

Sixth Korean National Health and Nutrition Examination Survey (2013-2015) is a nationwide, cross-sectional survey of the Korean population.

PATIENTS

Research subjects were selected by two-stage stratified cluster sampling of the population and housing census data. A total of 5639 subjects aged >18 years, who were not pregnant, and had undergone thyroid function testing during the survey period, were included.

MEASUREMENT

The level of serum TSH according to smoking status, iodine intake and presence of TPOAb were evaluated.

RESULTS

In the reference population, mean serum TSH level in current smokers (1.87 mIU/L, 95% CI, 0.52-5.37 mIU/L) was significantly lower than that in nonsmokers (2.33 mIU/L, 95% CI, 0.79-6.69 mIU/L, P < .001). The rate of thyroperoxidase antibody (TPOAb) positivity was higher in never smoker (7.7%) than past smokers (5.1%) and current smokers (4.7%), but sex-specific rate of TPOAb was not different according to smoking status. The lower serum TSH levels in current smokers were more apparent in iodine-deficient subjects (UIC < 100 μg/L), and this change was diminished in subjects with UICs between 100 and 299 μg/L. The difference in serum TSH levels in current smokers disappeared in subjects with UICs ≥ 300 μg/L.

CONCLUSIONS

Smoking is associated with a left-shift in serum TSH level that is more apparent in iodine-deficient subjects. Smoking status is not associated with the presence of TPOAb or iodine intake. The results suggest that smoking has a direct effect on thyroid function that is not mediated by autoimmune processes in the thyroid gland.