Limited Nrf2 activation and heterogeneous thyroidal effects in a 424-compound multi-assay screen call for rigorous testing of purported antioxidant and health-promoting supplements.

We screened 424 natural compounds, including many marketed as dietary supplements, to assess their antioxidant and thyroidal effects. In rat PCCL3 thyroid follicular cells, we performed parallel primary screens for antioxidant response element (ARE) activation, thyroglobulin (Tg) promoter activity, sodium/iodide symporter (NIS) promoter activity and iodide uptake. Hits were tested in downstream analyses, including ARE activation in Nrf2-knockout cells, an Nrf1 stabilization reporter, endogenous Nqo1, Tg and NIS mRNA and protein levels, and relationships between these readouts. Of 424 compounds, 69 induced ARE activity in wild-type cells; most also activated the ARE in Nrf2-knockout cells, and only 8 showed a profile consistent with predominantly Nrf2-dependent ARE activation. Several Nrf2-independent ARE inducers also stabilized an Nrf1-based reporter. Thirty compounds activated the Tg promoter and 26 significantly modulated iodide uptake, with only partial concordance between promoter activity, mRNA/protein expression and iodide uptake. Notable hits included genistein and apigenin, which showed balanced ARE and Tg induction together with NIS upregulation, supporting further evaluation in hypothyroidism and redifferentiation; luteolin, chrysin and diosmetin, which enhanced NIS expression and/or iodide uptake, highlighting potential as redifferentiation or adjunctive agents in thyroid cancer; emodin and gamma-oryzanol, which increased iodide uptake, suggesting utility in hypothyroidism or iodine deficiency; and bavachin, which showed the strongest iodide uptake induction without markedly increasing NIS protein, warranting mechanistic follow-up. Thyroid-relevant bioactivity was enriched among dietary supplements. Many compounds displayed complex Nrf2-dependent and -independent actions that can confer indication-specific thyroid benefits or risks, emphasizing the need for thyroid-focused evaluation of antioxidant and natural supplements.