Endocrine System and Syndromes

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 have an effect on specific target tissues. The targets have specific receptors for the hormones, like fitting a key to a lock. Sometimes the target tissue of a hormone is another gland. The hormone stimulates the receptor gland to produce and release another hormone into the blood, which in turn can affect different target tissues. An example of this is the hypothalamus gland 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 throughout the body.

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.

All of the endocrine glands are 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, stimulating 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, concentrations of the adrenal hormone cortisol are high in the morning and lower late in the evening, while levels of the pituitary hormones follicle-stimulating hormone (FSH) and lutenizing hormone (LH) increase and decrease in regular patterns to 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 epinephrine (adrenaline) from the adrenal glands in response to stress.