Endocrine-Disrupting Chemicals Affect Thyroid Hormone Activity in Pregnancy

Endocrine-Disrupting Chemicals Affect Thyroid Hormone Activity in Pregnancy
Endocrine-Disrupting Chemicals Affect Thyroid Hormone Activity in Pregnancy
The endocrine-disrupting chemical polychlorinated biphenyls may enter the placenta and affect thyroid hormone action during pregnancy.

The endocrine-disrupting chemicals polychlorinated biphenyls may enter the placenta during pregnancy and alter thyroid hormone activity at the cellular level, according to data published in the Journal of Clinical Endocrinology & Metabolism.

Polychlorinated biphenyls (PCBs) were banned in the United States in 1979 after being widely used in transformers and other electrical equipment as well as common household items like paint and tape. The chemicals, however, still make their way into the environment and many people have been exposed to low levels of the chemicals, according to a press release.

“As endocrine-disrupting chemicals, PCBs interfere with the way the thyroid hormone functions, but they don’t actually change the amount of the hormone found in the body,” study researcher R. Thomas Zoeller, PhD, of the University of Massachusetts in Amherst, said in the release.

“Although these effects are largely invisible in scientific studies that only judge thyroid activity by measuring hormone levels, they may be having a real impact on infants’ brain development.”

Zoeller and colleagues conducted a birth cohort study of 164 pregnant women with low-dose chemical exposure and no thyroid disease from the GESTE study.

For the study, the researchers measured maternal and cord blood thyroid hormone levels at birth. They quantified mRNA levels of CYP1A1 — an enzyme that converts endocrine-disrupting chemicals (EDCs) into a form that directly interferes with the body’s thyroid hormone receptors — and placental thyroid hormone receptor targets (placental lactogen and human growth hormone V [GH-V]).

Results showed that pregnancies with higher amounts of CYP1A1 in the placenta also appeared to have evidence of thyroid disruption in the tissue. These pregnancies also tended to have higher placental lactogen and GH-V levels, even as the mothers’ overall thyroid hormone levels remained the same, according to the release.

“Whatever is happening in the placenta likely reflects what is happening in the fetus,” Dr. Zoeller said. “To truly understand how endocrine-disrupting chemicals may be affecting pregnancies, the findings show we need to study not only hormone levels, but hormone activity at the cellular level.”

Zoeller also noted that smoking may exacerbate the effects of EDCs, as exposure to cigarette smoke increases the body’s production of CYP1A1. In the study, placental tissue from pregnant women who smoked generally contained higher levels of the enzyme, according to the release.


  1. Wadzinski TL et al. J Clin Endocrinol Metab. 2014;doi:10.1210/jc.2014-2629.