 Regulation of hormone synthesis of gonadal hormones, adrenocortical hormones, and thyroid hormones often depends on complex sets of direct-influence and feedback interactions involving the hypothalamic-pituitary-adrenal (HPA), -gonadal (HPG), and -thyroid (HPT) axes.Upon secretion, certain hormones, including protein hormones and catecholamines, are water-soluble and are thus readily transported through the circulatory system.Other hormones, including steroid and thyroid hormones, are lipid-soluble; to achieve widespread distribution, these hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes.