Exposure to phthalates during pregnancy and thyroid hormones in pregnant women and newborns

Abstract

Introduction: Phthalates are endocrine disrupting chemicals found in many consumer products. Prior evidence suggests that some phthalates may disrupt thyroid function, but the effect of phthalates on thyroid hormones in pregnant women and neonates has not been thoroughly investigated. Methods: We measured nine phthalate metabolites representing exposure to six parent phthalate diesters in urine collected at ~16 and 26 weeks gestation in a prospective pregnancy and birth cohort. We calculated the mean of 16 and 26 week metabolite concentrations and created exposure quartiles. We measured thyroid stimulating hormone (TSH), free and total thyroxine (FT4, TT4), and triiodothyronine (FT3, TT3) concentrations in maternal serum at ~16 weeks (n=184) and in cord serum (n=275). We estimated the associations of phthalates with maternal or cord thyroid hormones by multivariable linear regression. Results: Mothers in the 4th quartile of urinary monoethyl phthalate (MEP) had 0.05 ng/dL lower FT4 (95%CI -0.09, -0.01) compared to those in the 1st quartile (p-trend=0.02). Cord TSH was 19.8% lower among infants born to women in the 4th quartile of monobenzyl phthalate (MBzP) (95%CI: -34.1, -2.3), compared to those in the 1st quartile (p-trend 0.03). Infants born to mothers in the 4th quartile of di(2-ethylhexyl) phthalate (DEHP) metabolites had decreased cord FT3 [-0.09 pg/mL (95%CI: -0.19, 0.01); p-trend=0.04] versus those in the 1st quartile. We observed marginally significant inverse trends of both MEP (p-trend=0.06) and monoisobutyl phthalate (MiBP) (p-trend=0.06) with cord TT4. Conclusions: In this cohort, maternal urinary MEP was inversely associated with maternal FT4. Maternal urinary concentrations of MBzP, DEHP metabolites, MEP, and MiBP were inversely associated with cord thyroid hormones. These results suggest that maternal exposure to phthalates during pregnancy may disrupt maternal and cord thyroid hormones necessary for normal growth and neurodevelopment.