Research Shows the Importance of Chemical Mixtures on Thyroid Function

Research Shows the Importance of Chemical Mixtures on Thyroid Function (PDF) (1 page, 107K, About PDF)

Issue

Chemicals pervade our world - in the air we breathe, the water we drink, and the products we use. The U.S. Environ-mental Protection Agency constantly studies the many ways our bodies are affected by different types of chemi-cals. In one area, scientists are learning more about the risk of simultaneous exposure to multiple chemicals that disrupt thyroid hormones. A goal of this research is to protect the most sensitive populations, especially children and older adults, from adverse effects on the development of their nervous system.

Proper thyroid operation is essential for the developing child before and after birth, and for a number of physiological processes in adults. It's widely known in the scientific community that a number of individual chemicals on their own and at high concentrations can lead to disruption of thyroid activity. Relatively little is known, however, about how combinations of chemicals at natural exposure levels affect the thyroid. For example, are mixtures of such chemi-cals additive, synergistic or antago-nistic? In other words, do their effects simply combine, multiply upon combi-nation, or tend to cancel each other out?

Science Objective

A team of researchers with EPA's Office of Research and Development designed a study to test the additivity theory for a large mixture of poly-halogenated aromatic hydrocarbons (PHAH's), a family of chemicals that are known to disrupt thyroid activity. At low dosing, researchers concluded that the thyroid disrupting chemicals interacted in an additive manner. This suggests that while each individual chemical alone may not be enough to cause any noticeable effect, the cumula-tive affect of several chemicals could. At higher exposures, above those to which humans are normally exposed, researchers found a small synergistic effect on thyroid hormone disruption, meaning that the affects of the chemi-cals don't simply accumulate; essen-tially, they multiply by some new factor.

Application and Impact

Research analyzing the additive effect of thyroid disruptors is illustrating the importance of studying chemical mixtures and the potential impact on human health. This research is filling knowledge gaps and providing critical science needed to improve the ability to assess the risk of multiple chemi-cals that impact thyroid function. EPA research in this area falls under the Food Quality Protection Act (FQPA) of 1996 which mandates the assessment of risks that result from exposure to multiple, similar-acting chemicals.

Ongoing research will provide a broader understanding of the ways in which chemicals interact to disrupt the functioning of thyroid hormones, providing important information to environmental risk assessors and managers.

References

Crofton K.M.; Craft E.S.; Hedge J.M.; Gennings C.; Simmons J.E.; Carchman R.A.; Carter W.H. Jr; DeVito M.J. Thyroid-hormone-disrupting chemicals: evidence for dose-dependent additivity or synergism, Environ Health Perspect. 2005 Nov, 113(11):1549-54.

Teuschler L.; Klaunig J.; Carney E.; Chambers J.; Conolly R.; Gennings C.; Giesy J.; Hertzberg R.; Klaas-sen C.; Kodell R.; Paustenbach D.; Yang R. Support of science-based decisions concerning the evaluation of the toxicology of mixtures: a new beginning. Regul Toxicol Pharmacol. 2002 Aug, 36(1):34-9.

Wade M.G.; Parent S.; Finnson K.W.; Foster W.; Young-lai E.; McMahon A.; Cyr D.G.; Hughes C. Thyroid tox-icity due to subchronic exposure to a complex mixture of 16 organochlorines, lead, and cadmium. Toxicol Sci. 2002 Jun, 67(2):207-18.

Contact

Kevin M. Crofton, Ph.D., EPA's Office of Research and Development, U.S. Environmental Protection Agency, 919-541-2672, crofton.kevin@epa.gov