Persistent Organic Pollutants and Transthyretin-Bound Thyroxin in Plasma of Inuit Women of Childbearing Age

The Inuit population of Nunavik (Northern Quebec, Canada) is highly exposed to persistent organic pollutants (POPs) through their traditional diet. Some POPs, i.e., hydroxylated metabolites of polychlorinated biphenyls (OH-PCBs), pentachlorophenol (PCP), and perfluorooctane sulfonate (PFOS), compete with thyroxin (T4) for binding sites on transthyretin (TTR), a T4 transport protein found in plasma and cerebrospinal fluid. We tested the hypothesis that these TTR-binding compounds decrease circulating concentrations of T4 bound to TTR (T4-TTR) in Inuit women of reproductive age. We measured the concentration of T4-TTR in plasma samples obtained from 120 Inuit women (18-39 years old) by combining native-polyacrylamide gel electrophoresis and liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. Total T4, TTR, and thyroxin-binding globulin (TBG) concentrations were also determined, while POPs levels had been previously measured. The mean T4-TTR concentration was 8.4 nmol/L (SD = 2.4) with values ranging from 2.9 to 14.4 nmol/L. Linear regression analysis revealed that TTR, TBG, and total T4 concentrations were significant predictors (p < 0.002) of T4-TTR levels (total adjusted R-squared = 0.26, p < 0.0001) but not levels of OH-PCBs, chlorophenols, or PFOS. Our results suggest that circulating levels of these TTR-binding compounds in Inuit women of childbearing age are not high enough to affect TTR-mediated thyroid hormone transport. The possibility of increased delivery of these compounds to the developing brain requires further investigation.