Growth hormone is a vital hormone made up of around 190 amino acids, that regulates several important physiological processes such as growth and metabolism. It is secreted by cells found in the anterior pituitary called somatotrophs.
Growth hormone exerts its effects both directly and indirectly. The direct effects occur when the hormone acts on specific receptors present on cells. For example, fat cells referred to as adipocytes have receptors that are stimulated by growth hormone to break down triglyceride. The hormone also prevents these receptors from taking up circulating lipids.
Growth effects
The indirect effects are mediated by the effect of growth hormone on another hormone secreted by the liver called insulin-like growth factor-1 (IGF-1). The secretion of IGF-1 has a range of growth promoting effects when it acts on cellular targets. IGF-1 stimulates the growth of various tissues and also stimulates osteoblast and chondrocyte activity to increase bone growth. IGF-1 also plays a major role in muscle growth, promoting the differentiation and proliferation of a type of progenitor cell called a myoblast that gives rise to muscle cells. When myoblasts fuse, they form skeletal muscle fibers in which multiple nuclei can be observed due to each nucleus having originated from an individual myoblast.
Metabolic effects
Growth hormone plays an important role in the metabolism of lipids, proteins and carbohydrates. In some cases, these effects of growth hormone are direct, while in others IGF-1 is the key mediator. In other cases, the effects of growth hormone are both direct and indirect.
Lipid metabolism
Growth hormone promotes the use of lipids by promoting the breakdown of triglycerides and oxidation in cells.
Protein metabolism
In various different tissues, growth hormone enhances the uptake of amino acids and protein synthesis, as well as decreasing protein oxidation.
Carbohydrate metabolism
Growth hormone is involved in the regulation of blood glucose. It exerts anti-insulin activity by suppressing insulin’s ability to promote glucose uptake in the peripheral tissues. It also increases gluconeogenesis in the liver.
Growth hormone and disease
Both a deficiency and an excess of growth hormone are associated with disease states. These disorders may arise due to abnormalities in the pituitary gland, the hypothalamus or the hormone’s target cells.
Growth hormone deficiency may occur as a result of deficient production of the hormone or due to a deficient receptor response to the hormone. This can lead to delayed growth or dwarfism, although this depends on the age at which the deficiency started. The effect of too much growth hormone also depends on the age at which the deficiency developed. The two main conditions associated with an excess in growth hormone production are gigantism and acromegaly.
Gigantism and acromegaly
As growth hormone promotes the growth of bones, muscles and various other organs, too much of it causes abnormal growth of all these tissues, a condition that is referred to as gigantism when it applies to children and acromegaly when it applies to adults. Almost all cases are caused by a benign tumor in the pituitary gland referred to as an adenoma, although some rare tumors that arise in the lungs or pancreas can also secrete hormones that cause the pituitary gland to generate too much growth hormone. The effect in children is increased height, while adults develop bone deformities. Other common problems include visual disturbances, weakness and heart failure.
The conditions are diagnosed based on X-rays and blood tests, while computed tomography and magnetic resonance imaging are carried out to try and establish the cause. Treatment includes a combination of radiotherapy, surgery and medication to reduce the production of growth hormone.
Performance-enhancing effects
Synthetic human growth hormone (HGH) was approved fro use by the FDA in 1985 as a treatment for short stature and poor growth due to conditions such as Turner’s syndrome, Prader-Willi syndrome and growth hormone deficiency. However, the drug is most commonly used for reasons that are not FDA approved. Available widely over the internet, people often order HGH in combination with other drugs to build muscle and enhance their athletic performance, despite the effects on performance not being fully understood. Some researchers also believe that HGH can reverse the bodily deterioration that occurs as people age, although this effect is also not proven.
Possible side effects associated with the use of HGH include carpal tunnel syndrome, nerve or muscle pain, joint pain, increased cholesterol level, fluid accumulation (edema) and numbness or tingling in the skin.
Further Reading
Cadaveric pituitary-derived growth hormone delivery influences iatrogenic Alzheimer’s Disease