Before we discuss the decline of human growth hormone (hGH) with aging and what can be done about it, we think it would be helpful to review some physiology. hGH is produced in the pituitary gland by the somatotroph cells (hGH’s medical name is somatotropin). Under the influence of the hypothalamus (the part of the brain concerned with the more primitive bodily functions), hGH is released in spurts, predominantly at night during the third and fourth stages of deep sleep. As it circulates through the blood, hGH stimulates the production of insulin-like growth factor I (IGF-I) by the liver and other cells of the body. Because it is released in spurts, hGH is difficult to measure except in a research setting where blood can be drawn every 10 minutes. The blood level of IGF-I, in contrast, is more constant, and therefore, except under certain circumstances, it serves as a reliable surrogate measure of hGH production.
Interestingly, the use of growth hormone to treat Adult Growth Hormone Deficiency resulted from research on its use in two other disease states. Lack of hGH causes dwarfism or short-stature in children. In 1957, hGH isolated from human cadavers was injected into these children and normal growth ensued without significant side effects. However, when these children reached normal adult height, the hGH was discontinued because of its expense and scarcity (it took many human pituitary glands to make a few drops of the substance).
The other cause of hGH deficiency occurs when a person has had damage to his pituitary gland, either from surgery for a tumor of the gland or trauma. If this occurs when he is young, the patient will be growth-retarded just as the children mentioned above. If it occurs when he is past adolescence, he will have already grown to normal height, but usually will have other endocrine abnormalities such as cortisone, thyroid hormone, and sex steroid deficiencies. These latter hormones routinely have been replaced because their deficiencies can be immediately life threatening or at least decrease the quality of life in the short term; but since hGH was not thought to have any important physiologic role, other than causing growth in children, it was not routinely replaced.
The next section will show you what researchers found when they started to look at adults with hGH deficiency.
NEXT: hGH and Aging