Growth hormone | Definition, Function, Deficiency, & Excess

Summary

growth hormone (GH), also called somatotropin or human growth hormone, peptide hormone secreted by the anterior lobe of the pituitary gland. It stimulates the growth of essentially all tissues of the body, including bone. GH is synthesized and secreted by anterior pituitary cells called somatotrophs, which release between one and two milligrams of the hormone each day. GH is vital for normal physical growth in children; its levels rise progressively during childhood and peak during the growth spurt that occurs in puberty.

In biochemical terms, GH stimulates protein synthesis and increases fat breakdown to provide the energy necessary for tissue growth. It also antagonizes (opposes) the action of insulin. GH may act directly on tissues, but much of its effect is mediated by stimulation of the liver and other tissues to produce and release insulin-like growth factors, primarily insulin-like growth factor 1 (IGF-1; formerly called somatomedin). The term insulin-like growth factor is derived from the ability of high concentrations of these factors to mimic the action of insulin, although their primary action is to stimulate growth. Serum IGF-1 concentrations increase progressively with age in children, with an accelerated increase at the time of the pubertal growth spurt. After puberty the concentrations of IGF-1 gradually decrease with age, as do GH concentrations.

GH secretion is stimulated by growth hormone-releasing hormone (GHRH) and is inhibited by somatostatin. In addition, GH secretion is pulsatile, with surges in secretion occurring after the onset of deep sleep that are especially prominent at the time of puberty. In normal subjects, GH secretion increases in response to decreased food intake and to physiological stresses and decreases in response to food ingestion. However, some individuals are affected by abnormalities in GH secretion, which involve either deficiency or overabundance of the hormone.

GH deficiency is one of the many causes of short stature and dwarfism. It results primarily from damage to the hypothalamus or to the pituitary gland during fetal development (congenital GH deficiency) or following birth (acquired GH deficiency). GH deficiency may also be caused by mutations in genes that regulate its synthesis and secretion. Affected genes include PIT-1 (pituitary-specific transcription factor-1) and POUF-1 (prophet of PIT-1). Mutations in these genes may also cause decreased synthesis and secretion of other pituitary hormones. In some cases, GH deficiency is the result of GHRH deficiency, in which case GH secretion may be stimulated by infusion of GHRH. In other cases, the somatotrophs themselves are incapable of producing GH, or the hormone itself is structurally abnormal and has little growth-promoting activity. In addition, short stature and GH deficiency are often found in children diagnosed with psychosocial dwarfism, which results from severe emotional deprivation. When children with this disorder are removed from the stressing, nonnurturing environment, their endocrine function and growth rate normalize.

Children with isolated GH deficiency are normal in size at birth, but growth retardation becomes evident within the first two years of life. Radiographs (X-ray films) of the epiphyses (the growing ends) of bones show growth retardation in relation to the patients chronological age. Although puberty is often delayed, fertility and delivery of normal children is possible in affected women.

GH deficiency is most often treated with injections of GH. For decades, however, availability of the hormone was limited, because it was obtained solely from human cadaver pituitaries. In 1985, use of natural GH was halted in the United States and several other countries because of the possibility that the hormone was contaminated with a type of pathogenic agent known as a prion, which causes a fatal condition called Creutzfeldt-Jakob disease. That same year, by means of recombinant DNA technology, scientists were able to produce a biosynthetic human form, which they called somatrem, thus assuring a virtually unlimited supply of this once-precious substance.

Children with GH deficiency respond well to injections of recombinant GH, often achieving near-normal height. However, some children, primarily those with the hereditary inability to synthesize GH, develop antibodies in response to injections of the hormone. Children with short stature not associated with GH deficiency may also grow in response to hormone injections, although large doses are often required.

A rare form of short stature is caused by an inherited insensitivity to the action of GH. This disorder is known as Laron dwarfism and is characterized by abnormal GH receptors, resulting in decreased GH-stimulated production of IGF-1 and poor growth. Serum GH concentrations are high because of the absence of the inhibitory action of IGF-1 on GH secretion. Dwarfism may also be caused by insensitivity of bone tissue and other tissues to IGF-1, resulting from decreased function of IGF-1 receptors.

GH deficiency often persists into adulthood, although some people affected in childhood have normal GH secretion in adulthood. GH deficiency in adults is associated with fatigue, decreased energy, depressed mood, decreased muscle strength, decreased muscle mass, thin and dry skin, increased adipose tissue, and decreased bone density. Treatment with GH reverses some of these abnormalities but can cause fluid retention, diabetes mellitus, and high blood pressure (hypertension).

Excess GH production is most often caused by a benign tumour (adenoma) of the somatotroph cells of the pituitary gland. In some cases, a tumour of the lung or of the pancreatic islets of Langerhans produces GHRH, which stimulates the somatotrophs to produce large amounts of GH. In rare cases, ectopic production of GH (production by tumour cells in tissues that do not ordinarily synthesize GH) causes an excess of the hormone. Somatotroph tumours in children are very rare and cause excessive growth that may lead to extreme height (gigantism) and features of acromegaly.

Acromegaly refers to the enlargement of the distal (acral) parts of the body, including the hands, feet, chin, and nose. The enlargement is due to the overgrowth of cartilage, muscle, subcutaneous tissue, and skin. Thus, patients with acromegaly have a prominent jaw, a large nose, and large hands and feet, as well as enlargement of most other tissues, including the tongue, heart, liver, and kidneys. In addition to the effects of excess GH, a pituitary tumour itself can cause severe headaches, and pressure of the tumour on the optic chiasm can cause visual defects.

Because the metabolic actions of GH are antagonistic (opposite) to those of insulin, some patients with acromegaly develop diabetes mellitus. Other problems associated with acromegaly include high blood pressure (hypertension), cardiovascular disease, and arthritis. Patients with acromegaly also have an increased risk of developing malignant tumours of the large intestine. Some somatotroph tumours also produce prolactin, which may cause abnormal lactation (galactorrhea). Patients with acromegaly are usually treated by surgical resection of the pituitary tumour. They can also be treated with radiation therapy or with drugs such as pegvisomant, which blocks the binding of growth hormone to its receptors, and synthetic long-acting analogues of somatostatin, which inhibit the secretion of GH.

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Growth hormone | Definition, Function, Deficiency, & Excess