Growth Hormone Dysfunction and Endocrine Tissues of the Reproductive System
GROWTH HORMONE DYSFUNCTION
Hypothalamus-Pituitary Growth Hormone (GH/Somatotrophin) Axis
The hypothalamus releases growth hormone-releasing hormone (GHRH), which then stimulates the pituitary gland to produce GH. Secretion of GH by the pituitary into the bloodstream stimulates the liver to produce another hormone called insulin-like growth factor I (IGF-I). IGF-I is what actually causes tissue growth in the body. High levels of IGF-I, in turn, signal the pituitary to reduce GH production.
Somatostatin is another hormone made by the hypothalamus and it inhibits GH production and release. Normally, GHRH, somatostatin, GH, and IGF-I levels in the body are tightly regulated by one another and by sleep, exercise, stress, food intake, and blood sugar levels. If the pituitary continues to make GH independent of the normal regulatory mechanisms, the level of IGF-I continues to rise, leading to bone overgrowth and organ enlargement. High levels of IGF-I also cause changes in glucose and lipid metabolism, which can lead to diabetes, high blood pressure, and heart disease.
Growth hormone (GH) secreted by the anterior pituitary gland stimulates protein synthesis and cell division in cartilage and bone tissue. Gigantism results when excessive amounts of growth hormone are produced during childhood. Pituitary dwarfism occurs when too little growth hormone is produced and acromegaly occurs when too much GH is produced during adulthood.
GH-stimulated cell growth and protein synthesis occur via the release of somatomedins by the liver. This stimulation occurs almost immediately, when glucose and amino acid concentrations in the blood are elevated. A second effect appears hours later, as glucose and amino acid levels are declining. Under these conditions GH causes the breakdown of glycogen and lipid reserves, and directs peripheral tissues to begin using lipids instead of glucose as an energy source. As a result, blood glucose concentrations rise. These effects appear through an interaction between growth hormone and somatomedins.