Progress has been made in understanding the molecular basis of a number of clinical manifestations of thyroid disease, yet many questions remain. Why are there two thyroid hormone-receptor genes? Is the function of each of the two receptors indeed unique? How T3 receptors interact with other nuclear proteins and DNA-binding sites and how these interactions are influenced by T3 is incompletely understood. The developmental regulatory role of T3 receptor alpha 1 and its non-T3-binding alpha 2 variant needs to be defined. Most T3-regulated processes, especially those related to metabolism, muscle contraction, and brain development, function in concert with a number of other regulatory factors. The therapeutic applications of knowledge gained about the basic mechanisms of thyroid hormone action should ultimately extend beyond thyroid disease to processes regulated or influenced by T3; these include cardiac function, lipid metabolism, pituitary hormone secretion, and neural development.