Environmental Factor, July 2010, National Institute of Environmental Health Sciences
Fire Retardant Chemicals Linked to Lower TSH in Pregnancy
By Tara Ann Cartwright
According to a new study by NIEHS-funded researchers at the University of California, Berkeley (UCB) exposure to fire retardant chemicals is associated with lower levels of thyroid-stimulating hormone (TSH) during pregnancy.
UCB researcher Jonathan Chevrier, Ph.D., was first author on the study, which was published online this month in Environmental Health Perspectives. The principal investigator on the study was epidemiologist Brenda Eskenazi, Ph.D., director of the Center for Children's Environmental Health Research at the UCB School of Public Health, who presented a distinguished lecture March 16 at NIEHS on her work with farm workers in California exposed to pesticides (see story (https://factor.niehs.nih.gov/2010/april/science-pesticide.cfm)).
Eskenazi said of the new study, "Maternal thyroid hormones have been shown to cross the placenta and are essential for normal fetal growth and neurodevelopment, so our findings may have significant public health implications."
Discontinued chemicals persist in the environment
Polybrominated diphenylethers (PBDEs) are organobromine compounds, which have been used as fire retardants since the 1970s. Like other brominated flame retardants, PBDEs are a component in a wide array of consumer products such as electronics, toys, clothing, appliances, draperies, carpets, upholstery, and furniture foam.
The commercial mixtures penta-BDE and octa-BDE have been banned in Europe and several U.S. states because of health and safety concerns. The major North American manufacturer of these two PBDEs ceased production in 2004. However, PBDEs are still present in furniture and foam items manufactured before the ban and thus exposure is believed to be ongoing.
Since PBDEs are not chemically bound to the materials in which they are used, they are believed to be slowly and continuously released into the environment over time. Once released into the air, PBDEs can settle on soil or at the bottom of rivers and lakes. According to the authors, exposure to PBDEs is believed to occur primarily indoors, possibly through contact with house dust.
Health effects could be significant
Although the use of flame retardants is intended to reduce the risk of fire, as well as save lives and property, the potential health hazards of these synthetic chemicals have attracted increased scrutiny. According to Chevrier, in Europe, Asia, North America, and the Artic, traces of several PBDEs have been found in human breast milk, adipose tissue, and serum, and even in wildlife. Over the past three decades, the levels of PBDEs in human breast milk and serum have increased exponentially, and some experts believe they have not yet seen the highest levels.
The study's researchers said that evidence that PBDEs may affect human health is mounting. Animal studies have suggested that exposure to PBDEs impairs neurodevelopment, increases spontaneous motor behavior, and disrupts thyroid hormone (TH) homeostasis. Thyroid hormones are essential for normal fetal neurodevelopment, and human studies suggest that altered neurodevelopment may result from PBDE exposure during critical windows of development.
Taken together, this latest study allows officials from other U.S. states to evaluate the potential adverse health effects of human exposure to PBDEs and, perhaps, will help influence their support for legislative policies and programs to limit exposure to these global contaminants.
Citation: Chevrier J, Harley KG, Bradman A, Gharbi M, Sjödin A, Eskenazi B. 2010. Polybrominated diphenylether (PBDE) flame retardants and thyroid hormone during pregnancy. Environ Health Perspect. Epub before print. doi:10.1289/ehp.1001905. [Summary (https://factor.niehs.nih.gov/2010/july/science-extramural.cfm#two)]
Herbstman JB, Sjödin A, Kurzon M, Lederman SA, Jones RS, Rauh V, et al. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2866690/) 2010. Prenatal exposure to PBDEs and neurodevelopment. Environ Health Perspect 118(5):712-719.
Roze E, Meijer L, Bakker A, Van Braeckel KN, Sauer PJ, Bos AF. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2799472/) 2009. Prenatal exposure to organohalogens, including brominated flame retardants, influences motor, cognitive, and behavioral performance at school age. Environ Health Perspect 117(12):1953-1958.
(Tara Ann Cartwright, Ph.D., is a postdoctoral fellow in the NIEHS Laboratory of Neurobiology Membrane Signaling Group.)
Large Study Lends Power to Associations
The study is the largest yet to examine the relation between serum PBDE concentrations and thyroid function in pregnant women around the 26th week of gestation. Researchers measured the concentration of PBDEs, free thyroxine (T4), total T4, and thyroid-stimulating hormone (TSH) in serum samples collected from 270 pregnant low-income Latina women living in rural California. All participants were enrolled in a birth cohort study at the Center for the Health Assessment of Mothers and Children (CHAMACOS) (http://cerch.berkeley.edu/research-programs/chamacos-study), a birth cohort study that examines reproductive health and environmental exposures.
Researchers found an inverse association between the serum levels of the PBDE congeners BDE-28, 47, 99, 100 and 153 and thyroid-stimulating hormone (TSH). The odds of developing subclinical hyperthyroidism, defined as TSH below the normal range but normal T4, was also elevated in connection with increasing exposure to total PBDEs and BDE-100 and 153. The researchers detected PBDEs in all women.
"The impacts of hyperthyroidism in pregnancy may be profound as it has been associated with increased risks of miscarriage, premature birth, and intra-uterine growth retardation," the authors explained. The researchers added a caveat, "It is, however, important to note that the effects of maternal subclinical hyperthyroidism on fetal health are unclear and that more studies are needed on the subject in order to fully understand the implications of our findings."