Growth hormone deficiency is a condition that occurs when the pituitary gland fails to release enough growth hormone.
Growth hormone, also known as somatotropin, helps promote overall growth of the body, muscle mass development, protein synthesis, and carbohydrate and lipid metabolism.
So a deficiency in growth hormone, which can occur in kids and adults, results in problems in all of these areas.
Growth hormone is released by the pituitary which is a pea sized gland that is connected by a stalk to a part of the brain called the hypothalamus.
The hypothalamus secretes growth hormone releasing hormone which travels through a network of capillaries called the hypophyseal portal system.
Releasing hormone eventually reaches the anterior pituitary and triggers it to secrete growth hormone.
Growth hormone then travels via the blood to various target tissues in the body to stimulate growth. This is called the hypothalamic pituitary axis.
Normally, growth hormone is released in a pulsatile manner, throughout the day and peaks one hour after you fall asleep, but it is also secreted in response to various forms of internal and external stimuli.
For example, the hypothalamus senses when there’s hypoglycemia, or low blood sugar, and in response it secretes growth hormone releasing hormone.
Exercise causes the adrenal glands to secrete epinephrine and that stimulates the hypothalamus to react in the same way.
Estrogen and testosterone which are produced in high levels during puberty, stimulate the hypothalamus to release growth hormone releasing hormone as well.
The hypothalamic pituitary axis, however, is also regulated by three negative feedback loops which can prevent the release of additional growth hormone.
First off, high levels of growth hormone and growth hormone releasing hormone in the blood can signal the hypothalamus to stop secreting more growth hormone releasing hormone.
Second, it also signals the hypothalamus to produce somatostatin, which is a hormone that directly inhibits the anterior pituitary from releasing growth hormone.
Third, growth hormone secretion from the anterior pituitary is inhibited by a somatomedin called insulin-like growth factor 1, which also stimulates somatostatin release from the pituitary.
Somatomedins are produced in the liver and have many of the same growth-promoting effects on target tissues as growth hormone. It mediates these effects by binding to insulin-like growth factor 1 receptors as well as insulin receptors, which are present on almost all tissue types.
Growth hormone can stimulate growth in most organs in the body.
In the muscles, it stimulates amino acid uptake into the muscle cells, which helps with protein production and muscle growth.
Similarly, growth hormone stimulates the activity of osteoblasts in the bones and chondrocytes in the cartilage, which helps boost growth, particularly during the growth spurts of puberty.
Growth hormone also has many metabolic functions.
In adipose tissue, it causes lipolysis, which is the breakdown of fats, and gluconeogenesis in the liver, which is the production of glucose.
To increase glucose levels, growth hormone causes insulin resistance, which decreases insulin's ability to move glucose into the cells.
Growth hormone also triggers the steady release of insulin like growth factor from the liver, and once it’s released into the bloodstream, insulin like growth factor functions has many of the same effects as growth hormone.