Council on Environmental Health
Article Review

Prepared by: Jerome A. Paulson, MD
July 2008

Relationship of Thyroid Hormone Levels to Levels of Polychlorinated Biphenyls, Lead, p,p´- DDE, and Other Toxicants in Akwesasne Mohawk Youth

Lawrence M. Schell, Mia V. Gallo, Melinda Denham, Julia Ravenscroft, Anthony P. DeCaprio, and David O. Carpenter

Environ Health Perspect 116:806–813 (2008). doi:10.1289/ehp.10490 available via


  1. All people are exposed every day to mixtures of toxicants. Yet, most research looks at exposure to individual toxicants.
  2. There are a number of animal studies which raise concerns about exposures to persistent organic pollutants (POPs), particularly concern about interference with endocrine systems, including the thyroid system.
  3. Exposures to certain classes of POPs, such as polychlorinated biphenyls, always occurs in mixtures.
  4. Members of the Akwessane Mohawk Nation live on both sides of the St. Lawrence Seaway, an area heavily contaminated with PCBs, p,p´-dichlorophenyldichloroethylene (p,p´-DDE), a metabolite of p,p´- dichlorodiphenyltrichloroethane (p,p´-DDT), and hexachlorobenzene (HCB). In the past, they consumed large amounts of locally-caught fish and became contaminated with POPs.

Mother-child pairs were recruited when the children were between 10 and < 17 years old. Children were healthy. Blood was drawn and analyzed for the presence of lead, PCBs, p,p´-DDE, HCB, and mirex, and analyzed for cholesterol, triglycerides, TSH, T3, T4, and FT4. Multiple regression analysis was used to examine the effect of each toxicant on thyroid hormones when controlling for all other toxicants, as well as sex, age, triglycerides, cholesterol, breast-feeding, time of day when blood was collected, and duration of time between interview and blood draw.

Exposure to POPs have a variable effect on thyroid function. Exposure to the group of persistent PCBs was positively associated with TSH but inversely related to FT4. Nonpersistent PCBs were significantly and negatively related to FT4 only. HCB was negatively associated with T4, and lead was positively associated with T3. Breast-fed adolescents had higher levels of persistent PCBs and p,p´-DDE but not of nonpersistent PCBs or any other toxicant when compared with nonbreast-fed adolescents. Though having lower levels of persistent PCBs and p,p´-DDE, non-breastfed adolescents exhibited significant relationships between persistent PCBs and TSH and FT4, but breast-fed adolescents did not. It appears that PCBs from breast milk obscure the relationship between prenatal PCB exposure and thyroid function by adding random variation in PCB levels.

Exposure to PCB in-utero and early life influence thyroid function in children between 10 and 16 years later. This is not a straightforward relationship and is decreased by breastfeeding in infancy.

This study shows haw difficult studying mixtures can be and the difficulties with drawing conclusions about the impact of toxicants that were encountered in-utero as well as after birth.


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