Association between annual air pollutant exposure and thyroid function: a retrospective cross-sectional study

Despite the high burden of air pollutants, the association between air pollutant exposures and thyroid function in the general population is inconclusive. This study aimed to investigate the association between annual exposure to air pollutants and thyroid function in Korean adults.

This cross-sectional study included a total of 16,193 participants (8,812 males, 7,381 females; mean age: 53.8 ± 13.1 years) who underwent health examinations at Seoul National University and had thyroid-stimulating hormone (TSH) concentrations within the range of 0.1-10.0 mIU/L. The yearly average levels of five air pollutants, including particulate matter (PM_2.5 and PM₁₀), NO₂, SO₂, and CO, were estimated using the Community Multiscale Air Quality model. Multivariable linear and logistic regression models were used to examine the associations between annual ambient air pollutant exposures and thyroid function parameters, including TSH and free thyroxine (FT4).

Interquartile range (IQR) increases for PM_2.5, PM₁₀, NO₂, SO₂, and CO were positively associated with TSH and negatively associated with FT4 (all P < 0.001). Significant non-linear associations were observed for PM_2.5 and NO₂ with log-transformed TSH, whereas all air pollutants showed non-linear associations with log-transformed FT4. The IQR increases of all air pollutants showed positive associations with the highest quartile of TSH (high TSH) and the lowest quartile of FT4 (low FT4), respectively. When thyroid function was categorized into four groups based on high/low TSH and/or high/low FT4, significantly positive associations were demonstrated with high TSH-low FT4 group, with the strongest association observed for PM₁₀ (adjusted odds ratio [aOR], 1.55; 95% confidence interval [CI], 1.42-1.69), followed by SO₂ (1.38, 1.28-1.49), NO₂ (1.36,1.23-1.50), PM_2.5 (1.25, 1.17-1.34), and CO (1.23, 1.15-1.31). The sex-stratified analyses demonstrated more prominent associations among females.

Yearly exposure levels of air pollutants were positively associated with TSH, negatively associated with FT4 levels, and significantly associated with high TSH-low FT4. These findings suggest that exposure to ambient air pollution may contribute to thyroid function alterations at the population level, highlighting the importance of air quality improvement. Further longitudinal studies were warranted to consolidate these findings.