BACKGROUND: House dust contains many organic contaminants that can compete with the thyroid hormone (TH) thyroxine (T4) for binding to trans-thyretin (TTR). How these contaminants work together at levels found in humans and how displacement from TTR in vitro relates to in vivo T4-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 se-rum; 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 T4-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-T4 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 (Ki) was used to estimate inhibition levels of T4-TTR binding in human blood. RESULTS: The TTR-binding potency of the mixtures was well predicted by concentration addition. The ∼ 20% inhibition in FITC-T4 binding observed for the mixtures reflecting median concentrations in maternal and infant serum was extrapolated to 1.3% inhibition of T4-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 T4 levels in humans has been reported, so far. We hypothesize, however, that 1.3% inhibition of T4-TTR binding may ultimately be decisive for reaching a status of maternal hypothyroidism or hypothyroxinemia associated with impaired neurodevelopment in children.