Environmental tobacco smoke exposure in pregnant women and thyroid hormone levels in newborn cord blood: A birth cohort in China.

The impact of environmental tobacco smoke (ETS) exposure during pregnancy on neonatal thyroid hormones remains limited and inconclusive. This study aimed to evaluate the effects of ETS exposure on thyroid hormone levels in neonatal cord blood, using data from 591 mother-infant pairs. Maternal urinary cotinine (UC) concentrations, measured during the first, second, and third trimesters, served as biomarkers of ETS exposure. Levels of thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) levels were determined in neonatal cord blood samples. A generalized linear model assessed the associations between ETS exposure and thyroid hormones. Maternal UC detection rates were 84.94 %, 85.62 %, and 83.08 % for the first, second, and third trimesters, respectively. In the third trimester, maternal UC concentrations were negatively associated with cord blood TSH levels (β = -0.093, 95 % CI: -0.169, -0.014), and positively correlated with FT3 and FT4 levels (FT3: β = 0.023, 95 % CI: 0.009, 0.038; FT4: β = 0.091, 95 % CI: 0.057, 0.125). Stratified by infant sex, in girls, maternal UC concentrations in the third trimester was negatively correlated with cord blood TSH levels (β = -0.125, 95 % CI: -0.223, -0.026) and positively correlated with FT3 (β = 0.040, 95 % CI: 0.014, 0.066) and FT4 levels (β = 0.186, 95 % CI: 0.143, 0.229). In boys, the only significant association observed was a positive correlation between maternal UC concentrations in the third trimester and FT3 levels (β = 0.036, 95 % CI: 0.011, 0.060). These findings suggested that pregnant women are widely exposed to ETS, which may influence neonatal thyroid hormone levels. The third trimester may represent a sensitive window for ETS exposure affecting TSH, FT3, and FT4 levels, and these effects were particularly pronounced in female infants.