Growth hormone (GH or somatotropin) is a protein that generally stimulates body cells to increase in size and undergo more rapid cell division than usual. How GH accomplishes this action is not completely understood. It is known, however, that the hormone enhances the movement of amino acids through cell membranes and causes an increase in the rate at which cells convert these molecules into proteins.
GH also causes cells to decrease the rate at which they utilize carbohydrates and to increase the rate at which they use fats. The hormone's effect on amino acids seems to be the more important one.
Although the exact mechanism for controlling growth hormone secretion is unknown, it appears to involve two substances from the hypothalamus called growth hormone-releasingfactor (GRF) and growth hormone release-inhibiting factor (GIH or somatostatin).
A person's nutritional state also seems to play a role in the control of GH, for more of it is released during periods of protein deficiency and of abnormally low blood glucose concentration. Conversely, when blood protein and glucose levels are increased, there is a resulting decrease in growth hormone secretion.
Apparently the hypothalamus is able to sense changes in the concentrations of certain blood nutrients, and it releases GRF in response to some of them.
If growth hormone is not secreted in sufficient amounts during childhood, body growth is limited, and a type of dwarfism (hypopituitary dwarfism) results. In this condition, body parts are usually correctly proportioned and mental development is normal. However, an abnormally low secretion of growth hormone is usually accompanied by lessened secretions from other anterior lobe hormones, leading to additional hormone deficiency symptoms. For example, a hypopituitary dwarf often fails to develop adult sexual features unless hormone therapy is provided.
Hypopituitary dwarfism is sometimes treated by administering growth hormone, and this treatment may stimulate a rapid increase in height. The procedure, however, must be started before the epiphyseal disks of the person's long bones have become ossified. Otherwise growth in height is not possible.
An over secretion of growth hormone during childhood may result in gigantism— a condition in which the person's height may exceed 8 feet. Gigantism, which is relatively rare, is usually accompanied by a tumor of the pituitary gland. In such cases, various pituitary hormones in addition to GH are likely to be secreted excessively, so that a giant often suffers from a variety of metabolic disturbances and has a shortened life expectancy.
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| Gigantism |
If growth hormone is secreted excessively in an adult, after the epiphyses of the long bones have ossified, the person does not grow taller. The soft tissues, however, may continue to enlarge and the bones may become thicker. As a consequence, an affected individual may develop greatly enlarged hands and feet, a protruding jaw, and a large tongue and nose. This condition is called acromegaly, and like gigantism, it is often associated with a pituitary tumor.
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| Acromegaly patient |
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