growth hormone

growth hormone also called somatotrophin, is secreted from the anterior part of the pituitary gland. It is a major product of the gland, which contains 5 mg of the hormone (about 10% of its dry weight). As the name implies, growth hormone is important in controlling linear growth and, together with the thyroid hormones and the sex hormones, is important in determining the final height and development of an individual. It also has a role in controlling metabolism of foodstuffs, so that lack of the hormone in children results in short stature with the whole body in proportion, whereas deficiency in the adult results in weakness and depression.

Growth stops when the epiphyses (ends) of the bones fuse to the main shaft between them. Oversecretion before this occurs results in gigantism, whereas oversecretion afterwards results in acromegaly, a condition characterized by coarsening of the facial features and enlargement of the hands and feet. Interest in dwarfism, gigantism, and acromegaly has spanned the centuries; literature, especially for children, is filled with stories about dwarfs and giants, while Old Testament writings have several descriptions of giants. A study of paintings can also reveal subjects with disturbances of growth hormone secretion. A portrait from about 1365 bc of Tutankhamun's father-in-law illustrates some of the chacterisitics of acromegaly, but it was not until the late eighteenth century that Saucerette, a French surgeon, described a subject with features suggestive of this condition. During the nineteenth century a number of reports appeared and the term ‘acromegaly’ was coined in 1886 by Pierre Marie. In the following year, Minkowski (who also performed some of the early experiments important in the discovery of insulin) noted that acromegaly was associated with a pituitary tumour. Such tumours are now known to be the cause of gigantism and acromegaly. Once this was established, surgical treatment of the condition began to be attempted in the 1890s. In 1912 Cushing, a famous American neurosurgeon who also made major contributions to endocrinology, pioneered the technique of operating on pituitary tumours via the nasal route.

The nasal approach to the pituitary is possible because the gland itself lies in the midline at the base of the brain. Part of the visual pathway, the ‘optic chiasma’, lies in front of the pituitary, so a spreading tumour may lead to visual defects. This could explain why Goliath of Gath failed to see the pebble launched by David.

Growth hormone is a large peptide of 191 amino acids and is relatively species-specific, so only primate growth hormone is effective in man. This meant that until 1985, when it became possible to synthesize it, treatment of short stature employed growth hormone extracted from human pituitaries. As with some preparations of human gonadotrophins previously used in fertility treatment, some of the preparations were contaminated, leading to 1 in 1000 patients developing Creuzfeld Jacob disease, resulting in dementia and death. Currently biosynthetic growth hormone is employed.

Growth hormone is always detectable in the plasma of healthy individuals throughout life; it is not secreted continuously over the 24 hours, but in bursts. The most marked increase follows the onset of sleep, so there may be a basis for the old wives' tale that you will not grow if you do not get a good night's sleep. The hormone is present in the fetus, but does not appear to be necessary for growth until soon after birth. Its release is increased in puberty, at an earlier stage in girls than in boys. Secretion of growth hormone is controlled by the hypothalamus, a region of the brain which is important in regulating many functions including a major role in the response to stress: growth hormone is released in response to a number of stresses such as exercise, anaesthesia, and surgery. Prolonged stress may however suppress growth hormone release, so that children with marked emotional deprivation can show secondary growth failure. One such case is said to be Sir James Barrie who was short of stature and may have had some affinity with his creation, Peter Pan.

Release is stimulated in response to a rapid fall in blood glucose, which can be produced by an injection of insulin in a clinical test for growth hormone secretion. The hypothalamus controls growth hormone secretion by means of its own secretion of two peptides; somatostatin, which inhibits secretion, and growth hormone releasing hormone, which is stimulatory; these hormones reach the nearby anterior pituitary through local blood vessels.

Growth hormone stimulates the growth of the long bones, not directly but through the action of somatomedins, which are insulin-like growth factors made in the liver, and which also inhibit release of the hormone. It has a direct effect on metabolic processes throughout the body, supporting growth through enhanced formation of protein and nucleic acids (anabolic action) and of other constituents of lean body mass. By contrast its effects promote breakdown of carbohydrate and fat, with the energy released supporting growth. Because of the anabolic effects and because detection is difficult, growth hormone has been used by athletes to improve performance, although studies have shown it to be of little value.

Mary L. Forsling

See also development and growth; hypothalamus; peptides; pituitary gland.