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

Abstract

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.

Keywords: iodine; smoking; thyroid peroxidase antibodies; thyrotropin.