Background
Growth hormone (GH) is a protein-based polypeptide hormone. It stimulates growth and cell reproduction and regeneration in humans and other animals. It is a 191 amino acid, single-chain polypetide hormone that is synthesized, stored, and secreted by the somatotoph cells within the lateral wings of the anterior pituitary gland. Clinically, growth hormone is used to treat children's growth disorders and adult growth hormone deficiency.
In recent years, replacement therapies with human growth hormone (HGH) have become popular in the battle against aging and weight management. Reported effects include decreased body fat, increased muscle mass, increased bone density, increased energy levels, improved skin tone and texture, increased sexual function, and improved immune system function. At this time, HGH is still considered a very complex hormone and many of its functions are still unknown.
Growth hormone-releasing hormone and somatostatins released by neurosecretory nuclei of the hypothalamus into the portal venous blood surrounding the pituitary are the major controllers of GH secretion by somatotropes. HGH is synthesized and secreted from the anterior pituitary gland in a pulsatile manner throughout the day; surges of secretion occur at 3-5 hour intervals. The plasma concentration of GH during these peaks may range from 5-45 ng/mL. The largest and most predictable of these GH peaks occurs about an hour after onset of sleep. Otherwise, there is wide variation between days and individuals. Nearly 50% of HGH secretion occurs during the third and fourth REM sleep stages. Between the peaks, basal GH levels are low, usually less than 5 ng/mL for most of the day and night. Additional analysis of the pulsatile profile of GH described in all cases less than 1 ng/mL for basal levels while maximum peaks were situated around 10-20 ng/mL. A number of factors are known to affect HGH secret, such as age, gender, diet, exercise, stress, and other hormones. Young adolescents secrete HGH at the rate of about 700 µg/day while healthy adults secrete HGH at the rate of about 400 µg/day.
The most common disease of GH excess is a pituitary tumor composed of somatotroph cells of the anterior pituitary. These somatotroph adenomas are benign and grow slowly, gradually producing more and more GH. In children, growth failure and short stature are the major manifestations of GH definciency, with common causes including genetic conditions and congenital malformations. Adults with GH deficiency present with non-specific problems including truncal obesity with a relative decrease in muscle mass and, in many instances, decreased energy and quality of life. Diagnosis of GH deficiency involves a multi-step diagnostics process, usually culminating in GH stimulation tests to see if the patient's pituitary gland will release a pule of GH when provokes by various stimuli.
