Thyroid hormones are important regulators of vertebrate growth and development, and are under the control of the hypothalamic-pituitary-thyroid axis. Nuclear thyroid receptors (TRs), which act as inducible transcription factors, mediate cellular functions of thyroid hormones. The molecular structure of several subtypes of TRs have been elucidated in vertebrate species, including N-terminal truncations as well as C-terminal variations in the domain responsible for binding hormone. In this paper, we review current information on the thyroid receptors studied in the vertebrate species with emphasis on recent findings in goldfish concerning functional significance of the thyroid receptor subtypes.