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Growth hormone (GH) is a hormone that stimulates growth and cell reproduction in humans and other animals. It is a 191-amino acid, single chain polypeptide hormone which is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior pituitary gland. The hormone is also known as somatotrophin when referring to growth hormone produced by recombinant DNA technology, and is abbreviated "rhGH".

Regulation

Peptides released by neurosecretory nuclei of the hypothalamus into the portal venous blood surrounding the pituitary are the major controllers of GH secretion by the somatotropes. However, although the balance of these stimulating and inhibiting peptides determines GH release, this balance is affected by many physiological stimulators and inhibitors of GH secretion.

Stimulators of GH secretion include

• growth hormone releasing hormone (GHRH) from the arcuate nucleus
• low levels of blood sugar (hypoglycemia)
• exercise
• ghrelin
• dietary protein
• arginine
• estradiol

Inhibitors of GH secretion include

• circulating concentrations of GH and IGF-1 (negative feedback)
• somatostatin from the periventricular nucleus
• glucocorticoids
• dietary carbohydrate

Secretion patterns

The amount and pattern of GH secretion change throughout life. Basal levels are highest in early childhood. The amplitude and frequency of peaks is greatest during the pubertal growth spurt. Healthy children and adolescents average about 8 peaks per 24 hours. Adults average about 5 peaks. Basal levels and the frequency and amplitude of peaks decline throughout adult life. Also may have impact/help growth of colon, lung, and breast cancer.

Most of the physiologically important secretion occurs as several large pulses or peaks of GH release each day. The plasma concentration of GH during these peaks may range from 5 to 35 ng/mL or more. Peaks typically last from 10 to 30 minutes before returning to basal levels. 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. Between the peaks, basal GH levels are low, usually less than 3 ng/mL for most of the day and night.

Functions of GH

Effects of growth hormone on the tissues of the body can generally be described as anabolic (building up). Like most other protein hormones GH acts by interacting with a specific receptor on the surface of cells.

Stimulating the increase in height in childhood is the most widely known effect of GH, and appears to be stimulated by at least two mechanisms.

• GH also stimulates production of insulin-like growth factor 1 (IGF-1, formerly known as somatomedin C), a hormone homologous to proinsulin. The liver is a major target organ of GH for this process, and is the principal site of IGF-1 production. IGF-1 has growth-stimulating effects on a wide variety of tissues. Additional IGF-1 is generated within target tissues, making it apparently both an endocrine and an autocrine/paracrine hormone. IGF-1 also has stimulatory effects on osteoblast and chondrocyte activity to promote bone growth.
• GH directly stimulates division and multiplication of chondrocytes of cartilage. These are the primary cells in the growing ends (epiphyses) of children's long bones (arms, legs, digits).

In addition to increasing height in children and adolescents, growth hormone has many other effects on the body:

• Increases muscle mass through the sarcomere hyperplasia
• Increases calcium retention, and strengthens and increases the mineralization of bone
• Increases protein synthesis
• Promotes lipolysis
• Stimulating the growth of all internal organs excluding the brain
• Reduces liver uptake of glucose
• Plays a role in fuel homeostasis
• Promotes gluconeogenesis in the liver
• It stimulates the immune system
• It contributes to the maintenance and function of pancreatic islets

Treatments unrelated to deficiency

GH treatment improves muscle strength and slightly reduces body fat in Prader-Willi syndrome, which are significant concerns beyond the need to increase height. GH is also useful in maintaining muscle mass in wasting due to AIDS. GH can also be used in patients with short bowel syndrome to lessen the requirement for intravenous total parenteral nutrition.

Uses that are controversial include:

• GH treatment to enhance weight loss in obesity
• GH treatment for remission of Multiple sclerosis
• GH treatment to reverse effects of aging in older adults (see below)
• GH treatment for Crohn's disease and ulcerative colitis
• GH treatment for fibromyalgia
• GH treatment for idiopathic short stature
• GH treatment for Crohn's disease and ulcerative colitis
• GH treatment for bodybuilding or athletic enhancement

Anti-aging agent

A Stanford University School of Medicine survey of clinical studies on the subject published in early 2007 showed that the application of GH on healthy elderly patients increased muscle by about 2 kg and decreased body fat by the same amount. However, these were the only positive effects from taking GH. No other critical factors were affected, such as bone density, cholesterol levels, lipid measurements, maximal oxygen consumption, or any other factor that would indicate increased fitness. Researchers also didn't discover any gain in muscle strength, which led them to believe that GH merely let the body store more water in the muscles rather than increase muscle growth. This would explain the increase in lean body mass. Regular application of GH did show several negative side effects such as joint swelling, joint pain, carpal tunnel syndrome, and an increased risk of diabetes.

Claims for GH as an anti-aging treatment date back to 1990 when the New England Journal of Medicine published a study where GH was used to treat 12 men over 60. At the conclusion of the study all the men showed statistically significant increases in lean body mass and bone mineral.

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