Relative Effect Potency Estimates of Dioxin-like Activity for Dioxins,Furans, and Dioxin-like PCBs in Adults Based on Two Thyroid Outcomes

Background: Toxic equivalency factors (TEFs) are an important component in the risk assessment of dioxin-like human exposures. At present, this concept is based mainly on in vivo animal experiments using oral dosage. Consequently, the current human TEFs derived from mammalian experiments are applicable only for exposure situations in which oral ingestion occurs. Nevertheless, these “intake” TEFs are commonly—but incorrectly—used by regulatory authorities to calculate “systemic” toxic equivalents (TEQs) based on human blood and tissue concentrations, which are used as biomarkers for either exposure or effect.Objectives: We sought to determine relative effect potencies (REPs) for systemic human concentrations of dioxin-like mixture components using thyroid volume or serum free thyroxine (FT₄) concentration as the outcomes of interest.Methods: We used a benchmark concentration and a regression-based approach to compare the strength of association between each dioxin-like compound and the thyroid end points in 320 adults residing in an organochlorine-polluted area of eastern Slovakia.Results: REPs calculated from thyroid volume and FT₄ were similar. The regression coefficient (β)-derived REP data from thyroid volume and FT₄ level were correlated with the World Health Organization (WHO) TEF values (Spearman r = 0.69, p = 0.01 and r = 0.62, p = 0.03, respectively). The calculated REPs were mostly within the minimum and maximum values for in vivo REPs derived by other investigators.Conclusions: Our REPs calculated from thyroid end points realistically reflect human exposure scenarios because they are based on chronic, low-dose human exposures and on biomarkers reflecting body burden. Compared with previous results, our REPs suggest higher sensitivity to the effects of dioxin-like compounds.