Development of TEFs for PCB congeners by using an alternative biomarker — Thyroid hormone levels

Polychlorinated biphenyls (PCBs) are ubiquitous toxic contaminants. Health risk assessment for this class of chemicals is complex: the current toxic equivalency factor (TEF) method covers dioxin-like (DL-) PCBs, dibenzofurans, and dioxins, but excludes non-DL-PCBs. To address this deficiency, we evaluated published data for several PCB congeners to determine common biomarkers of effect. We found that the most sensitive biomarkers for DL-non-ortho-PCB 77 and PCB 126 are liver enzyme (e.g., ethoxyresorufin-O-deethylase, EROD) induction, circulating thyroxine (T4) decrease, and brain dopamine (DA) elevation. For DL-ortho-PCB 118 and non-DL-ortho-PCB 28 and PCB 153, the most sensitive biomarkers are brain DA decrease and circulating T4 decrease. The only consistent biomarker for both DL- and non-DL-PCBs is circulating T4 decrease. The calculated TEF-TH, based on the effective dose to decrease T4 by 30% (ED30) with reference to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is identical to both TEF-WHO98 and TEF-WHO05 for TCDD and DL-PCBs (correlation coefficients are r = 1.00, P < 0.001; and r = 0.99, P < 0.001, respectively). We conclude that T4 decrease is a prospective biomarker for generating a new TEF scheme which includes some non-DL-congeners. The new TEF-TH parallels the TEF-WHO for DL-PCBs and, most importantly, is useful for non-DL-PCBs in risk assessment to address thyroid endocrine disruption and potentially the neurotoxic effects of PCBs.