The endocrine system is an integrated network that consists of various glands located throughout the body. Together with the nervous system, the endocrine system controls and regulates many internal bodily functions. While the nervous system uses nerve impulses as a means of control, the endocrine system uses chemical messenger molecules called hormones. Hormones are produced, stored, and secreted by the network of glands. When hormones are released by the endocrine glands into the bloodstream, they target specific cells, tissues, or organs. Each target has specific receptors for the hormones, like fitting a key to a lock.
There are several parts of this network. One, the hypothalamus, is an endocrine gland located in the lower middle are of the brain. Another, the pituitary, is located in its own place just below the hypothalamus and inside the sella turcica. Signals from the brain prompt the hypothalamus to produce several different hormones that stimulate or inhibit the pituitary. These signals cause the pituitary to increase or decrease the hormones it produces and releases into the bloodstream. These hormones that are released from the pituitary in varying amounts travel through the bloodstream to endocrine glands such as the thyroid, adrenal glands, testicles and ovaries (gonads). Several other organs and tissues throughout the body are also hormone targets.
Most of the endocrine glands are controlled by feedback systems that avoid hormone imbalance. For example, the hypothalamus stimulates the pituitary gland and then the adrenal gland to regulate its function. The hypothalamus first releases corticotropin-releasing hormone (CRH), which causes the pituitary gland to release corticotropin (more commonly called ACTH or adrenocorticotrophic hormone). ACTH in turn stimulates the adrenal gland to produce cortisol. Once cortisol reaches a certain threshold, the hypothalamus and pituitary will decrease CRH and ACTH production, creating a negative feedback loop.
Some hormones, such as cortisol, have a daily or monthly pattern of release. The level of cortisol is high in the morning and lower late in the evening. Levels of pituitary hormones called 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.
Because the endocrine system and the network of glands are often interdependent, a disorder that affects one gland may cause disease related to other glands in the system. For example, a disorder that affects the hypothalamus may affect the pituitary as well as other target organs that are "downstream." Endocrine syndromes can be categorized by the gland that is affected:
- Primary disorders affect target organs (e.g., thyroid, adrenal glands).
- Secondary disorders affect the gland that regulates the target organ (usually the pituitary gland).
- Tertiary disorders arise from the hypothalamus.
To learn more about specific endocrine glands and the hormones they produce, see the Summary of Endocrine Glands section.