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Member Research & Reports

Member Research & Reports

Brown: Increased Exposure to Endocrine-Disrupting Chemicals Associated with Decreased Thyroid Hormones in Pregnant Women and Newborns

Phthalates, endocrine-disrupting chemicals that are commonly found in consumer products, may adversely affect thyroid hormones, but findings from prior epidemiologic studies are inconsistent.

[Photo: Dr. Megan Romano]

In a prospective cohort study, conducted by lead author Dr. Megan Romano, adjunct assistant professor of epidemiology at the Brown University School of Public Health and assistant professor of epidemiology at the Dartmouth Geisel School of Medicine, researchers investigated whether maternal urinary phthalate metabolite concentrations and phthalate mixtures measured during pregnancy were associated with thyroid hormones among pregnant women and newborns.

Concentrations of nine phthalate metabolites [monoethyl phthalate (MEP), mono-n-butyl phthalate, mono-isobutyl phthalate, monobenzyl phthalate (MBzP), and four monoesthers of di(2-ethylhexyl) phthalate] were quantified in urine collected at approximately 16 and 26 weeks’ gestation among women in the Health Outcomes and Measures of the Environment Study (2003–2006, Cincinnati, Ohio). Thyroid stimulating hormone (TSH) and free and total thyroxine and triiodothyronine were measured in maternal serum at 16 weeks’ gestation (n = 202) and cord serum at delivery (n = 276). Using multivariable linear regression, the researchers assessed associations between individual urinary phthalate metabolites and concentrations of maternal or cord serum thyroid hormones. Weighted quantile sum regression (WQS) was used to create a phthalate index describing combined concentrations of phthalate metabolites and to investigate associations of the phthalate index with individual thyroid hormones.

Results indicated that with each 10-fold increase in 16-week maternal urinary MEP, maternal serum total thyroxine (TT4) decreased by 0.52 μg/dL (95 percent CI: −1.01, −0.03). For each 10-fold increase in average (16- and 26-week) maternal urinary MBzP, cord serum TSH decreased by 19 percent (95 percent CI: −33.1, −1.9). Among mothers, the phthalate index was inversely associated with maternal serum TT4 (WQS beta = −0.60; 95 percent CI: −1.01, −0.18). Among newborns, the phthalate index was inversely associated with both cord serum TSH (WQS beta = −0.11; 95 percent CI: −0.20, −0.03) and TT4 (WQS beta = −0.53; 95 percent CI: −0.90, −0.16).

These findings suggest that co-exposure to multiple phthalates was inversely associated with certain thyroid hormones (TT4 in pregnant women and newborns, and TSH in newborns) in this birth cohort. These findings highlight the need to study chemical mixtures in environmental epidemiology.

This article was published in The International Journal of Hygiene and Environmental Health, March 2018

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