INTRODUCTION
Growth
Hormone is a polypeptide hormone. This means it is composed of a
long chain of amino acids, 191 to be exact.
Under normal physiologic conditions, growth hormone is secreted
by the anterior pituitary gland. This is a gland that lies at the
base of the brain in a bony cavity called the Sella Turcica. In
addition to growth hormone, the anterior pituitary also secretes
prolactin, thyroid stimulating hormone, luteinizing hormone, follicle
stimulating hormone, and adrenal corticotropic hormone.
The secretion of growth hormone by the pituitary gland is initiated
by the hypothalamus, another gland in the brain that lies right
next to the pituitary. The hypothalamus initiates growth hormone
secretion by secreting growth hormone releasing hormone (GHRH);
at the same time it stops secreting a growth hormone inhibitory
hormone called somatostatin. When somatostatin is turned off and
GHRH is turned on, the pituitary will release growth hormone in
bursts of activity. These bursts of growth hormone release occur
primarily during deep stages of sleep, such as stage 3 and stage
4.
Once released in the blood, growth hormone is very short lived.
It is generally completely metabolized and gone within a half-hour.
During that time, however, it manages to reach the liver and many
other cells in the body, and induce them to make another polypeptide
hormone called Insulin-like Growth Factor One (IGF-1). It is really
IGF-1 that travels around to the various tissues of the body to
effect most of the benefits that we attribute to growth hormone.
The secretion of growth hormone itself is regulated by a classic
biofeedback loop. This means when levels of growth hormone in the
blood reach a certain threshold, growth hormone stimulates receptors
in the pituitary to stop further growth hormone secretion.
It also stimulates receptors in the hypothalamus to stop GHRH and
turn on somatostatin. IGF-1, which goes up in response to growth
hormone, also feeds back on the pituitary and hypothalamus to help
control growth hormone secretion. This is nature's system of checks
and balances to assure we don't have too much of any one hormone.
View Cart
