Cl-PFESA disrupts thyroid hormone secretion in human thyrocytes and nonmonotonic effects of iodide co-exposure.

Chlorinated polyfluoroalkyl ether sulfonic acids (Cl-PFESAs, F-53B) are widespread environmental pollutants that have been introduced as emerging alternatives to perfluorooctane sulfonate. They are suspected to have thyroid-disrupting effects, however, their impact on human thyroid hormone (TH) synthesis and the underlying mechanisms remain poorly understood. Using Cl-PFESAs at concentrations relevant to children's serum levels, in the present study we found that Cl-PFESA mixture significantly elevated thyroxine (T4) and triiodothyronine (T3) synthesis in human thyroid follicular epithelial cells (Nthy-ori 3-1)-contrasting with the hypothyroid effect reported in animal studies. We further demonstrated that mechanistically Cl-PFESAs activated transcription factor paired box gene 8 (PAX8) at environmentally relevant concentrations, upregulating thyroglobulin, thyroid peroxidase, and catalase while downregulating sodium iodide symporter. PAX8 knockdown completely abolished Cl-PFESA-induced upregulation of thyroid-specific transcription factors and TH hypersecretion, confirming that PAX8 plays a central role in mediating the TH-stimulating effect of Cl-PFESA. Furthermore, co-exposure with iodide exhibited a nonmonotonic effect: low-to-medium iodide levels (100-300 μg/L) enhanced Cl-PFESA-stimulated TH secretion, whereas high levels (≥600 μg/L) suppressed it. Taken together, this study revealed a novel, human thyrocyte-specific hyperthyroid effect of Cl-PFESA at children serum relevant concentrations and a PAX8-mediated molecular mechanism. Our findings provide crucial toxicological evidence for the human health risk that this class of emerging PFAS alternatives may pose and its potential complications by iodine intake status.