Contaminants that are present in the Arctic aquatic food chain may alter brain development in Inuit infants. One possible mechanism involves interference with the thyroid system during the developmental stage by metabolites of persistent organic pollutants that resemble the endogenous hormone L-thyroxine (T4). The disruption of T4 transport has received special attention since some hydroxylated metabolites of polychlorinated biphenyls and polybrominated diphenyl ethers have been shown to bind to transthyretin (TTR), a transport protein involved in supplying the foetus with T4 and transporting T4 across the blood brain barrier to the developing brain. Recently, several other compounds have been identified as binders of TTR and thyroid binding globulin (TBG), another T4 transport that may be involved in supplying the placenta with T4. The objective of this study is to examine whether plasma levels of these compounds are linked to a decrease in the proportion of circulating T4 bound to TTR and/or TBG in Inuit women of childbearing age. Such a relation would strengthen the biological plausibility of a link between exposure to food chain contaminants and adverse effects on brain development in this population. Major compounds that displace T4 from its transport proteins will also be identified.