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

Abstract

BACKGROUND: House dust contains many organic contaminants that can compete with the thyroid hormone (TH) thyroxine (T 4 ) 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 T 4 -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 TTRbinding potency to in vivo inhibition levels of T 4 -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-T 4 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 (K i ) was used to estimate inhibition levels of T 4 -TTR binding in human blood. RESULTS: The TTR-binding potency of the mixtures was well predicted by concentration addition. The ∼ 20% inhibition in FITC-T 4 binding observed for the mixtures reflecting median concentrations in maternal and infant serum was extrapolated to 1.3% inhibition of T 4 -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 TTRbinding compounds and circulating T 4 levels in humans has been reported, so far. We hypothesize, however, that 1.3% inhibition of T 4 -TTR binding may ultimately be decisive for reaching a status of maternal hypothyroidism or hypothyroxinemia associated with impaired neurodevelopment in children. https://doi.

Human Exposure to Chlorinated Paraffins via Inhalation and Dust Ingestion in a Norwegian Cohort

“… 10 , 11 For example, concentrations of polybrominated diphenyl ethers (PBDEs) in indoor air have been found to be several orders of magnitude higher than in outdoor air. 12 , 13 Inhalation and ingestion of contaminated air and dust are two major exposure pathways for SVOCs, 14 − 17 leading to elevated body burden and potential adverse health effects 18 , 19 including immunosuppression, 20 diabetes, 21 and asthma. 22 …”

Per- and polyfluoroalkyl substances (PFASs) in Swedish household dust and exposure of pet cats

“…A recent study evaluated dust-related thyroid hormone disrupting compounds in mixtures corresponding to household dust, infant blood serum, and adult blood serum (Hamers et al 2020 ). In total, 25 chemicals were measured for their binding potency to the transport protein TTR in a competitive in vitro assay (FITC-T4).…”

Removing Critical Gaps in Chemical Test Methods by Developing New Assays for the Identification of Thyroid Hormone System-Disrupting Chemicals—The ATHENA Project

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