Thyroid Hormones (T4, T3)

Abstract

Thyroid hormone is produced by all vertebrates. In mammals, the thyroid gland is derived embryologically from endoderm at the base of the tongue and develops into a bilobed structure lying anterior to the trachea. The structure and arrangement of thyroid tissue, however, vary significantly among species. Several key transcription factors, thyroid transcription factors 1 and 2 (TTF 1 and 2) and Pax8, are required for normal thyroid gland development and regulate gene expression in the adult thyroid gland. The thyroid gland receives a rich blood supply, as well as sympathetic innervation, and is specialized to synthesize and secrete thyroxine (T4) and triiodothyronine (T 3) into the circulation (Fig. 1). This process is regulated by thyroid-stimulating hormone ([TSH], or thyrotropin) secreted from the pituitary, which is, in turn, stimulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. Both TSH and TRH are regulated in a negative-feedback loop by circulating T4 and T3. Iodine and the trace element selenium are essential for normal thyroid hormone metabolism. © 2005 Humana Press Inc.