Disruption of thyroid homeostasis by thyroid disrupting pesticides is the initial, critical effect that may lead to harm to the developing nervous system. Thus, in place of the rat developmental neurotoxicity study, the special study described in this document entails a mechanistic approach to generate specific data on the thyroid (i.e., the primary target of the chemical of interest) to protect the developing nervous system from thyroid hormone disrupting chemicals. The specific purpose of this special study is to generate data to establish NOAELs and LOAELs (or benchmark doses) that may be used to derive reference does (RfDs) that would be protective of the ability of a chemical to disrupt thyroid function in pregnant females and in the fetus and newborn. This special study will generally be requested based on the results of a study(ies) in adult animals that provide evidence that a pesticide affects thyroid function or structure.

• Guidance for Thyroid Assays in Pregnant Animals, Fetuses and Postnatal Animals, and Adult Animals (PDF)(12 pp, 38 K)