A link between phthalates -- industrial chemicals used to make plastics such as polyvinyl chloride -- and thyroid hormone levels has been confirmed by University of Michigan researchers in the first large-scale and nationally representative study of phthalates and bisphenol A (BPA) in relation to thyroid function in humans (Environmental Health Perspectives, July 11, 2011).
The study authors also reported suggestive findings consistent with a previously reported link between BPA and thyroid hormone levels. BPA is a synthetic, manmade chemical with estrogenic properties that is used in a variety of products, including dental sealants. In animal studies, exposure to BPA early in life has been shown to prompt various abnormalities in the early development of the brain, behavior, prostate gland, and breast tissues.
The researchers used publicly available data from the U.S. National Health and Nutrition Examination Survey to compare urine metabolites and serum thyroid measures from 1,346 adults and 329 adolescents. Generally speaking, greater concentrations of urinary phthalate metabolites and BPA were associated with greater impacts on serum thyroid measures, according to lead study author John Meeker, ScD, an assistant professor of environmental health sciences at the university's School of Public Health.
He and his colleagues found an inverse relationship between urinary markers of exposure and thyroid hormone levels, meaning as urinary metabolite concentrations increased, serum levels of certain thyroid hormone levels decreased.
The study findings showed the strongest relationship between thyroid disruption and DEHP, a phthalate commonly used as a plasticizer. Research has shown that the primary exposure to DEHP is through diet. Urine samples in the highest 20% of exposure to DEHP were associated with as much as a 10% decrease in certain thyroid hormones compared with urine samples at the lowest 20% of exposure.
"This seems like a subtle difference," Meeker said, "but if you think about the entire population being exposed at this level you'd see many more thyroid-related effects in people."
While the study focused primarily on adults, these findings underscore the need for more research on adults, pregnant women, and children, Meeker said, because fetal and child development may be particularly vulnerable to disruptions in thyroid hormone levels associated with exposure to environmental chemicals.
Meeker pointed out that the study had limitations. Since urine and serum samples were collected at a single point in time, the researchers couldn't conclude a cause-and-effect relationship; it would be better to follow people over time and collect several samples, especially since these chemicals metabolize quickly and one snapshot may not represent the true chemical exposure.
The researchers have several ongoing studies on the potential impacts of phthalate and BPA exposure on pregnancy outcomes and child development.
