Testing, Assessment and Classification of Thyroid Hormone Disruptors – Flash Report

Perturbation of thyroid hormone levels during gestation and the early post-natal period may cause neurodevelopmental impairment. The evaluation of effects in offspring is therefore relevant for the toxicological assessment of chemicals affecting thyroid function, as reflected in current ECHA/EFSA guidance on the identification of endocrine disruptors (ED).

Following the logic of this guidance, substances meet the ED criteria if they elicit adverse effects, have endocrine activity and if the adverse effects and endocrine activity are linked by an endocrine mode of action (MoA) unless the MoA is not relevant for humans. However, clear regulatory guidance on the assessment of whether substances meet the ED criteria for the thyroid modality considering developmental neurotoxicity (DNT) as an adverse outcome is currently unavailable. This is particularly relevant as a large proportion of substances evaluated under different legislation are shown to affect the thyroid, many through mechanisms not of human relevance.

Accordingly, this symposium had the objective of reviewing the state of the science of thyroid hormone disruption mediated DNT, and its current assessment in regulatory decision-making.