Chemical selection for the Thyroid Validation Study coordinated by EURL ECVAM and involving EU-NETVAL laboratories.

The aim of the Thyroid Validation Study, coordinated by EURL ECVAM and involving EU-NETVAL laboratories, was to validate selected non-animal methods for the identification of chemicals that can potentially disrupt the thyroid hormone system in humans. The validation study was organized in two parts: Part 1 was to assess method performance and develop standard operating procedures, where needed, and Part 2 was to assess the mechanistic relevance of the methods using a set of vali­dation chemicals. This paper describes the stepwise process to select this validation set of chemicals, mainly based on extensive literature review and expert judgment elicitation to identify chemicals for which there was evidence to show their (lack of) ability to perturb the thyroid hormone signaling mechanisms or modes of action covered by the methods. A unique contribution of the study lies in its mechanistic coverage of molecular targets within the thyroid gland but also regulatory mechanisms in peripheral tissues, reflecting a multifaceted perspective on thyroid hormone action. The validation set consisted of 30 chemicals, providing a balanced representation across a broad chemical space and offering insights into the mechanistic relevance of the selected methods. Once validated, these methods will contribute to advancing the identification and evaluation of endocrine disruptors, informing regulatory decisions, and promoting alternative testing strategies.