Hormones are signals or chemical messengers released from endocrine glands in the body. Endocrine glands such as the thyroid, pituitary, adrenal, pancreatic glands, the testes and the ovaries are ductless glands that secrete the hormones they produce directly into the bloodstream.

From there, the hormones travel to their various different sites of action in the body where they either stimulate or inhibit certain functions.

Some of the signals which trigger the release of hormones include:

Hormones from other endocrine glands - Most hormones are released as a result of another hormone being released. For example, thyrotropin releasing hormone (TRH) triggers the release of thyroid stimulating hormone (TSH), which, in turn, triggers the release of thyroxine from the thyroid gland.

Blood composition - For example, insulin is released by the pancreatic beta cells in response to a rise in blood sugar after a meal. Insulin then helps the body utilize the blood sugar. The pancreas is also stimulated to release the hormone glucagon when the blood sugar falls too low.

Nerve stimulation - Certain nerve impulses may cause the adrenal gland to release hormones such as adrenalin in response to stress, for example.

Feedback system

M

ost hormones are regulated by a negative feedback system which prevents excess secretion of the hormone. For example, when there is an excess of the thyroid hormone thyroxine in the blood, the pituitary stops releasing TSH. Ordinarily, TSH stimulates the thyroid gland to secrete thyroxine.

However, the inhibition of TSH release regulates this secretion. This method of inhibiting a hormone's own release is called feedback inhibition. Other hormones such as estrogen, progesterone, testosterone and cortisol are also regulated by negative feedback inhibition.

A feedback system may also be positive. For example, as the labor contractions increase during childbirth, the hormone oxytocin is stimulated to be released in higher amounts. Similarly, during lactation, more nursing causes an increase in milk production.

Hormone signals

The signalling activity of hormones often involves the following:

• Mediating the synthesis of other hormones by stimulating their manufacture and release
• Stimulating the transport of hormones to target cells so they can mediate their actions
• The signal from the hormone may be conveyed by second messengers within the cells
• Finally, once the hormone has mediated its action, it is degraded in order to terminate the action