Growth Hormone Physiology

Overview
  • Growth Hormone (GH) is a peptide hormone secreted by the anterior pituitary and is critical for normal growth to adult size and functions by coordinating multiple metabolic processes.
Regulation
  • Regulatory Axis
    • GH is synthesized and secreted by somatotrophs of the anterior pituitary. Synthesis and secretion of pituitary GH is under the control of the hypothalamus via releasing and inhibitory hormones. Hypothalamic GHRH acts as the releasing hormone whereas hypothalamic somatostatin acts as an inhibitory hormone although control by GHRH is likely the dominant control mechanism. Once released, GH induces hepatic synthesis of a second powerful endocrine peptide hormone known as Insulin-like Growth Factor-1 (IGF-1). Relatively stable levels of GH are achieved via negative feedback control as both GH and IGF-1 serve to inhibit synthesis of hypothalamic GHRH and pituitary GH as well as stimulate synthesis of somatostatin.
  • Pattern
    • Growth Hormone is secreted throughout an individual's lifetime although levels increase throughout childhood, peak during puberty, and slowly decline with age. The pubescent peak of GH release may be a contributing factor to the adolescent growth spurt. On a daily basis, GH levels display a cyclical pattern with peaks and troughs occurring every few hours although typically an especially large peak occurs early during sleep. However, a variety of acute inducers of GH release are known with the most influential being hypoglycemia although stress and protein-energy malnutrition can also result in increased GH release.
Function
  • Overview
    • Growth Hormone is a powerful metabolic hormone, stimulating growth of nearly all body tissues and enhancing lean body mass. Although many of these functions are the direct result of GH on tissues, a large contribution also occurs indirectly via synthesis of IGF-1 which displays powerful metabolic effects. Therefore, the physiological effects of GH described below should be considered the combined actions of both GH and IGF-1.
  • Growth
    • As mentioned, GH induces growth in nearly any tissue that can possibly grow and is required for reaching full size and weight of a variety of organs. However, its most important growth-promoting effect is on cartilage and bone which occurs with cooperation from Thyroid Hormones. Together, these hormones are critical for proper linear growth.
  • Metabolic Effects
    • GH is an important modulator of total body metabolism and promotes tissue protein deposition, utilization of body fat, and reduces utilization of body sugars. The effects of GH on protein and lipid metabolism result in GH being a key factor maintaining lean body mass. Most importantly, however, GH reduces utilization of sugars by promoting resistance of body tissues to insulin, thus resulting in increased blood glucose concentration.