Smoking Associated With Lower Serum Thyroid-Stimulating Hormone Levels

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The mechanism behind the relationship between smoking and abnormalities in the thyroid is not clearly defined.
The mechanism behind the relationship between smoking and abnormalities in the thyroid is not clearly defined.

Smoking is associated with reduced serum thyroid-stimulating hormone (TSH) levels, according to a study recently published in Clinical Endocrinology.

Although smoking is believed to increase the risk for Graves ophthalmopathy, goiter, Graves disease, and abnormalities in thyroid hormones, the precise manner in which smoking affects the thyroid gland remains unclear. 

People with iodine deficiencies may experience an antithyroid effect, whereas those with sufficient iodine may experience immunogenic effects. This study sought to investigate the link between serum TSH levels and both smoking status and iodine levels.

Study researchers included 5639 individuals from the Sixth Korean National Health and Nutrition Examination Survey, a cross-sectional study performed in South Korea between 2013 and 2015. All participants were aged ≥ 18 years, had been tested for thyroid function in the survey, and were not pregnant. The outcomes of this study were intake of iodine, serum TSH levels and their relation to both smoking status and urine iodine concentration (UIC), and levels of thyroperoxidase antibody (TPOAb).

Smokers in the study presented with significantly lower levels of serum TSH than nonsmokers (1.87 mIU/L, 95% CI, 0.52-5.37 mIU/L in smokers vs 2.33 mIU/L, 95% CI, 0.79-6.69 mIU/L in nonsmokers; P<.001). These lower levels of serum TSH were more prominent in individuals with an iodine deficiency with a UIC <100 μg/L, though a UIC of 100 to 299 μg/L served to reduce this discrepancy.

In current smokers, the discrepancy in levels of serum TSH became indistinguishable in UICs of at least 300 μg/L. Although there was no sex-related difference in TPOAb rate in relation to smoking status, never-smokers showed higher rates of TPOAb positivity (7.7%) than current smokers (4.7%) and previous smokers (5.1%).

Researchers conclude that 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.” 

Reference

Park S, Kim WG, Jeon MJ, et al. Serum thyroid-stimulating hormone levels and smoking status: data from the Korean National Health and Nutrition Examination Survey VI [published online March 31, 2018]. Clin Endocrinol. doi: 10.1111/cen.13606

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