Fetal Thyroid Function and In Utero Exposure to Fine Particle Air Pollution: a Birth Cohort Study

Abstract

Background: Thyroid hormones are critical for fetal development and growth. Currently, there is no information available whether prenatal exposure to fine particle air pollution (PM2.5) affects fetal thyroid function and what the impact is on birth weight in normal healthy pregnancies. Methods: We measured the levels of free thyroid hormones (FT3, FT4) and thyroid stimulating hormone (TSH) in cord blood (n=545) and maternal blood (n=472) of mother-child pairs enrolled in the ENVIRONAGE birth cohort. Daily PM2.5 exposure levels were calculated for each participant’s residence. Results: Prenatal PM2.5 exposure during pregnancy did not affect maternal thyroid hormones, whereas an interquartile range (IQR) increment in entire pregnancy PM2.5 exposure changed cord blood TSH levels by -11.0% (95% CI: -18.6, -2.8; p=0.01). The FT4/FT3 ratio was negatively associated with an IQR increment of entire pregnancy PM2.5 exposure (-41.7%; -62.8, -20.6; p=0.0001), which was mainly attributed to a reduction in FT4 concentrations in cord blood. The estimated effect of an IQR increment in third trimester PM2.5 exposure on birth weight was mediated for 26% (-23 g; -42, -4; p=0.02) by decreasing cord blood FT4 levels. Conclusions: In normal healthy pregnancies, exposure to PM2.5 air pollution affects fetal thyroid function. Cord blood FT4 mediates to some extent the association between PM2.5 exposure and birth weight. Whether PM2.5-induced alterations in fetal thyroid function has bearing on pathological consequences later in life requires further elucidation.