Transthyretin-Binding Activity of Complex Mixtures Representing the Composition of Thyroid-Hormone Disrupting Contaminants in House Dust and Human Serum

BACKGROUND: House dust contains many organic contaminants that can compete with the thyroid hormone (TH) thyroxine ([Formula: see text]) for binding to transthyretin (TTR). How these contaminants work together at levels found in humans and how displacement from TTR in vitro relates to in vivo [Formula: see text]-TTR binding is unknown. OBJECTIVES: Our aims were to determine the TTR-binding potency for contaminant mixtures as found in house dust, maternal serum, and infant serum; to study whether the TTR-binding potency of the mixtures follows the principle of concentration addition; and to extrapolate the in vitro TTR-binding potency to in vivo inhibition levels of [Formula: see text]-TTR binding in maternal and infant serum. METHODS: Twenty-five contaminants were tested for their in vitro capacity to compete for TTR-binding with a fluorescent FITC- [Formula: see text] probe. Three mixtures were reconstituted proportionally to median concentrations for these chemicals in house dust, maternal serum, or infant serum from Nordic countries. Measured concentration–response curves were compared with concentration–response curves predicted by concentration addition. For each reconstituted serum mixture, its inhibitor–TTR dissociation constant ([Formula: see text]) was used to estimate inhibition levels of [Formula: see text]-TTR binding in human blood. RESULTS: The TTR-binding potency of the mixtures was well predicted by concentration addition. The [Formula: see text] inhibition in FITC- [Formula: see text] binding observed for the mixtures reflecting median concentrations in maternal and infant serum was extrapolated to 1.3% inhibition of [Formula: see text]-TTR binding in maternal and 1.5% in infant blood. For nontested mixtures reflecting high-end serum concentrations, these estimates were 6.2% and 4.9%, respectively. DISCUSSION: The relatively low estimated inhibition levels at median exposure levels may explain why no relationship between exposure to TTR-binding compounds and circulating [Formula: see text] levels in humans has been reported, so far. We hypothesize, however, that 1.3% inhibition of [Formula: see text]-TTR binding may ultimately be decisive for reaching a status of maternal hypothyroidism or hypothyroxinemia associated with impaired neurodevelopment in children. https://doi.org/10.1289/EHP5911