A comprehensive study on the association of personal air pollution exposure with thyroid hormones in pregnant women

Over 10 epidemiological studies have investigated air pollutants' association with thyroid hormone levels in pregnant women, but none have used personal air pollutant exposure data or included benzene, toluene, ethylbenzene, and xylene (BTEX). We assessed the relationship between personal exposure to nitrogen dioxide (NO₂), particulate matter ≤2.5 µm (PM_2.5), and BTEX with maternal thyroid hormones during pregnancy.

The study relied on 437 pregnant women from the SEPAGES cohort (Grenoble, France, 2014-2017). Personal exposure to NO₂, PM_2.5, and BTEX was measured 1 week before blood sampling (mean: 19th gestational week), where thyroid hormones (free and total triiodothyronine (FT3, TT3), thyroxine (FT4, TT4), thyroid-stimulating hormone, and the TT3/TT4 ratio) were assessed. We linearly regressed air pollutants against individual thyroid hormone concentrations, adjusting for covariates, including urinary iodine concentration. Ambient NO₂ and PM_2.5 exposures were also analyzed for comparison.

Personal PM_2.5 was positively associated with TT4 (β = 2.3%; 95% confidence interval [CI] = 0.4%, 4.2%, per doubling of exposure) and FT4 (β = 1.9%; 95% CI = -0.3%, 4.2%). On the contrary, negative associations between doubling the ethylbenzene exposure and TT4 (β = -1.2%; 95% CI = -2.4%, 0.1%) and FT4 levels (β = -1.3%; 95% CI = -2.7%, 0.1%) were observed. The association between ambient PM_2.5 1 week before hormone measurement and FT4 was consistent with the association observed for personal exposure. Associations between ambient PM_2.5 and TT4 were positive but attenuated across all examined time windows.

Personal short-term exposure estimates provide evidence of a potential association between PM_2.5, ethylbenzene, and altered thyroxine (TT4 and FT4) levels in pregnant women, warranting replication in other populations and larger samples.