Endocrine control of growth.

Human growth is a complex process starting at conception and completing in adolescence at the time of growth plate fusion. Growth can be divided into four phases: (1) fetal, where the predominant endocrine factors controlling growth are insulin and the insulin-like growth factors. (2) Infancy, where growth is mainly dependent upon nutrition. (3) Childhood, where the growth hormone-insulin-like growth factor-I (GH-IGF-I) axis and thyroid hormone are most important. (4) Puberty, where along with the GH-IGF-I axis the activation of the hypothalamo-pituitary-gonadal axis to generate sex steroid secretion becomes vital to the completion of growth. GH is released from the pituitary in a pulsatile fashion under the control of GHRH, Ghrelin, and somatostatin and, via a complex signal transduction cascade, initiates the release of IGF-I within many tissues but predominantly the liver and at the growth plate. IGF-I acts in an autocrine and paracrine manner via the IGF-I receptor to stimulate cell proliferation and longitudinal growth. Activation of the pituitary-gonadal axis during puberty occurs via a complex interaction of factors including kisspeptin, leptin, gonadotrophin releasing hormone, and tachykinin ultimately leading to augmentation of GH secretion, the pubertal growth spurt, and fusion of the growth plates. Many other hormones can affect the GH-IGF-I system or directly affect cell proliferation at the growth plate including thyroid hormone, vitamin D, and corticosteroids.