Bisphenol A linked to lower thyroid hormone levels in newborn boys
By Bono Sen
A new study, funded in part by NIEHS, reports, for the first time, that exposure to bisphenol A (BPA) during pregnancy may affect thyroid hormone levels in pregnant women and newborn boys.
The study by researchers at the University of California (UC), Berkeley, published online Oct. 4 in the journal Environmental Health Perspectives, adds to the growing health concerns about BPA. In the background section of their study, the authors cited earlier research that found detectable levels of BPA in the urine of 95 percent of U.S. women of reproductive age in testing conducted as part of the National Health and Nutrition Examination Survey (NHANES) 2007-2008.
“Our data showed no evidence that there is a safe level of exposure," said lead author Jonathan Chevrier, Ph.D., research epidemiologist at UC Berkeley's Center for Environmental Research and Children's Health (CERCH). “Studies suggest that small changes in thyroid hormone level, even if they’re within normal limits, may still have an effect on brain development. ”
BPA exposure affects thyroid hormone levels
The researchers analyzed BPA levels in the urine samples of 476 women during the second half of pregnancy. Also, blood samples to measure thyroid hormone levels were taken from 364 of moms and newborns during pregnancy and within a few days of birth, respectively. The study participants were residents of California’s Salinas Valley, a low-income community of mostly Mexican-American farm workers, who were part of the Center for the Health Assessment of Mothers and Children of Salinas (CHAMACOS) study. BPA was detected in the urine of 82 percent of women.
For newborn boys, but not girls, a 9.9 percent decrease in thyroid stimulating hormone was observed with every doubling in BPA level. This association was strongest when the women’s BPA levels were measured during the third trimester, and the time between BPA and thyroid hormone measurements was shorter. According to the authors, this temporal pattern may be due to a specific developmental window of susceptibility to BPA, or may suggest a transient effect of BPA on thyroid hormones.
Clues as to why this response was not seen among newborn girls may be found in some animal studies that suggest that female rats have higher levels of a BPA metabolizing enzyme than their male counterparts.
BPA as a public health concern
BPA is a chemical widely used in making products such as baby and water bottles, tubing and medical devices, sports equipment, household electronics, plastic toys, lining of cans, and thermal receipts.
A previous NIEHS-funded study shows that BPA and phthalates are associated with a range of adverse health outcomes, many of which may be mediated through altered thyroid signaling. Thyroid hormones play an essential role in prenatal and postnatal growth and brain development in humans. Disruption of thyroid function may be responsible for potential effects of BPA on cognition and behavior.
Currently, 11 U.S. states have banned BPA in some products. In July, the U.S. Food and Drug Administration (FDA) banned the chemical from baby bottles and sippy cups. It has not banned BPA from food packaging yet, because research is still ongoing. NIEHS grantees are collaborating with the National Toxicology Program and FDA to help develop state-of-the art rodent studies on BPA. Information gathered from these studies will be used to inform regulatory agencies tasked to determine the safety of BPA-containing consumer products.
In addition to NIEHS support for CHAMACOS and CERCH, the study was also supported by funding from the U.S. Environmental Protection Agency, UC Institute for Mexico and the United States, and the UC Berkeley Center for Latino Policy Research.
Citation: Chevrier J, Gunier RB, Bradman A, Holland NT, Calafat AM, Eskenazi B, Harley KG. 2012. Maternal Urinary Bisphenol A During Pregnancy and Maternal and Neonatal Thyroid Function in the CHAMACOS Study. Environ Health Perspect; doi:10.1289/ehp.1205092 [Online 4 October 2012].
(Bono Sen, Ph.D., is the program director for health literacy for the NIEHS journal Environmental Health Perspectives.)