Growth hormone, also known as somatotropin, is a hormone which helps regulate the rate of growth in the body.
Secretion of growth hormone is dependent on the hypothalamic-pituitary axis.
The hypothalamus, which is a part of the brain, secretes growth hormone-releasing hormone into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior, or front part of the pituitary gland.
In the anterior pituitary, there are many different types of cells, each responsible for producing a type of hormone.
The growth hormone-releasing hormone binds to a surface protein on one of these cells, called somatotroph cells, and stimulates them to release of growth hormone.
Normally, growth hormone releasing 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 secrete growth hormone releasing hormone as well.
Also, during puberty, increased levels of estrogen and testosterone stimulate the hypothalamus to release growth hormone releasing hormone, which is responsible for the growth spurt.
Now there are a few negative feedback loops that generally control the release of growth hormone.
First, increased levels of growth-hormone-releasing hormone in the blood signals the hypothalamus to stop making more.
Second, when growth hormone reaches tissues like the liver, muscles, and bones, they make somatomedins, which are hormones that signal the anterior pituitary to stop producing growth hormone.
Third, growth hormone and somatomedins together signal to the hypothalamus to produce somatostatin, which is also called the growth hormone inhibiting hormone.
Somatostatin acts like a lawyer that tells other endocrine organs to “cease and desist” in producing more hormones.