The development of in vitro models that preserve the functional characteristics of the normal thyroid gland has been a challenging objective of recent experimental thyroidology. A major aim of such endeavors has been to facilitate the development of simple, reliable, reproducible testing strategies for compounds interacting with, and perturbing the function of, the thyroid follicular cell (TFC). The earliest experimental thyroid models were based on organ culture or tissue slice preparations or, alternatively, short-term cell suspensions. Subsequently, however, it has become possible to maintain TFCs as monolayer cultures in which a high level of thyroid-specific differentiation and responsiveness may be preserved over a prolonged period. Such characteristics, which allow experimental intervention and the subsequent study of cell function and morphology, have facilitated the development of biological assays for thyrotrophin (TSH) (1) and thyroid autoantibodies (2) in serum, and have recently begun to encompass applications in the field of cellular toxicology, where the application of these new investigative tools has enabled the identification of the sites and mechanisms of action of agents demonstrating direct thyroid toxicity in vivo (3).