Adverse outcome pathway network-based approach for the evaluation of thyroid hormone system disruption for human health and environmental regulatory hazard assessment.

Thyroid hormones (THs) are essential for growth, development, and metabolism across vertebrates. Thyroid hormone system disruption (THSD), as one of the mechanisms of endocrine disruption (ED), is widely recognised as a critical concern for both human and environmental health. Important challenges remain in the availability and regulatory application of methods for THSD assessment. Here we discuss recent methodological advances in the area of THSD, including validation efforts for in vitro assays and THSD-sensitive endpoints as additions to existing Organisation for Economic Co-operation and Development (OECD) Test Guidelines using fish early-life stages. We propose a modular adverse outcome pathway (AOP) network-based framework integrating mechanistic and effect-level information to support regulatory hazard assessment for both human and environmental health under a One Health paradigm. The framework summarises essential elements of the THSD AOP network relevant to human and environmental health and methods were mapped to these mechanism and effect components. In a case study on resorcinol, a chemical of regulatory interest relevant for both human and environmental health, we demonstrate how emerging methods, including in vitro and fish eleutheroembryo (developmental stage between hatching and the onset of independent feeding) assays as new approach methods (NAMs), complement traditional data to strengthen weight-of-evidence for ED classification. By mapping gaps and priorities, this work advances the integration of NAMs into chemical safety assessment, supporting a future update of OECD Guidance Document 150. These efforts align with European Union initiatives to gradually phase out animal testing for chemical hazard assessment and implement Next Generation Risk Assessment.