Thyroid hormone (TH) plays a critical role in development, growth, and cellular metabolism. TH production is controlled by a complex mechanism of positive and negative regulation. Hypothalamic TSH-releasing hormone (TRH) stimulates TSH secretion from the anterior pituitary. TSH then initiates TH synthesis and release from the thyroid gland. The synthesis of TRH and TSH subunit genes is inhibited at the transcriptional level by TH, which also inhibits posttranslational modification and release of TSH. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback at the pituitary was thought to be the primary regulator of serum TSH levels. However, study of transgenic animals showed an unexpected, dominant role for TRH in regulating the hypothalamic-pituitary-thyroid axis and an unanticipated involvement of the thyroid hormone receptor ligand-dependent activation function (AF-2) domain in TH negative regulation. These results are summarized in the review.