A thyroid toxicant workshop sponsored by the National Toxicology Program Center for the Evaluation of Risks to Human Reproduction convened on 28-29 April 2003 in Alexandria, Virginia. The purpose of this workshop was to examine and discuss chemical-induced thyroid dysfunction in experimental animals and the relevance of reproductive and developmental effects observed for prediction of adverse effects in humans. Presentations highlighted and compared reproductive and developmental effects of thyroid hormones in humans and rodents. Rodent models of thyroid system dysfunction were presented. Animal testing protocols were reviewed, taking into account protocol designs that allow extrapolation to possible human health effects. Potential screening methods to assess toxicant-induced thyroid dysfunction were outlined, and postnatal bioassays of thyroid-related effects were discussed.