What is the endocrine system?
The endocrine system is made up of various glands located throughout the body. Together with the nervous system, it controls and regulates all bodily functions. While the nervous system uses nerve impulses as a means of control, the endocrine system uses chemical messenger molecules called hormones. These hormones are released by the endocrine glands into the blood stream, where they seek out specific target tissues. The targets have receptors that accept the hormones like fitting a key to a lock. Some of the hormones’ targets are other glands – they are secreted by one gland and travel to another, where they stimulate the production and secretion of another hormone that then takes action. An example of this is the hypothalamus gland (see table on Tests page) that releases thyrotropin-releasing hormone (TRH). This hormone stimulates the pituitary gland to release thyrotropin (more commonly called TSH or thyroid-stimulating hormone). TSH in turn stimulates the thyroid gland to produce thyroid hormones thyroxine (T4) and triiodothyronine (T3), which help to regulate the rate of metabolism.
To learn more about specific endocrine glands, the hormones they produce, and the disorders associated with their improper function, see the table of endocrine glands on the Tests page.
All of the endocrine glands are normally carefully controlled with the use of feedback systems. For example, the amount of thyroid hormone in the bloodstream acts as a regulating factor on the hypothalamus and pituitary, telling them to release more TRH and TSH respectively when thyroid hormone concentrations in the blood decrease. In some cases, such as thyroid hormone, the body strives to keep a relatively constant amount in the blood.
Some hormones have a daily or monthly pattern of release. For example, cortisol (produced by the adrenal glands) concentrations are high in the morning and lower late in the evening, while both follicle-stimulating hormone (FSH) and luteinising hormone (LH) (produced by the pituitary gland) increase and decrease with and regulate a woman’s monthly menstrual cycle. Other hormones are generally present in very small quantities in the blood and are released in specific situations, such as the release of adrenaline (epinephrine) from the adrenal glands in response to stress.