This document reviews the state of assays for thyroid toxicants across four vertebrate classes (fish, amphibians, birds and mammals) within the context of a thorough review of thyroid endocrinology across these taxa. Considering the similarity of the endocrinology of the thyroid gland across these taxa, a major feature of this document is the integration of assay comparisons across vertebrates in the final chapter. By assessing the state of the assays amongst mammalian (rodent), fish, amphibian, and avian species, the state of thyroid assays, redundancies, and information gaps presented themselves.