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February 28, 2018.
What is the endocrine system?

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.

Accordion Title
About the Endocrine System and Syndromes
  • Causes of Endocrine Syndromes

    Hormones affect numerous systems throughout the body, including development of male and female sexual characteristics, fertility, growth, energy consumption, digestion, glucose utilization, stress response, fluid/water balance, bone metabolism, and maintenance of proper blood pressure. When glands produce too much or too little of a specific hormone and affect these processes, it is known as a hormone imbalance. Some of these conditions have names, such as Cushing syndrome (associated with excess cortisol) because they are linked to a typical set of symptoms and complications.

    Endocrine gland dysfunction may occur when there is a problem with the gland itself, a problem in the feedback system, and/or due to a lack of response by the hormone's target tissues. A decrease in hormone production may be related to trauma, damage by the immune system, infection, crowding of the hormone-producing cells by a tumor, or an inherited gene mutation that affects the quantity, quality, or structure of a hormone. The failure of a gland to produce and release enough hormone to stimulate the target gland to produce and release its hormone may also decrease production.

    Increased hormone production may be related to a feedback system imbalance such as the pituitary gland producing too much of a hormone such as ACTH and disrupting the feedback system. Increased production may be related to hyperplasia (enlarged glands) or a tumor of the hormone-producing cells, but also can occur due to lack of tissue response, medication use, or an inherited condition.

    Endocrine tumors that produce excess hormones are generally small and usually benign. Most of them are located inside the affected gland and produce a single type of hormone. Rarely, they may be cancerous. It is also very rare that endocrine-disrupting tumors may be located elsewhere in the body. A tumor may cause symptoms because of the excess hormone it is producing, because its growth crowds out and decreases the production of other hormones in the gland, or because its physical size presses against surrounding nerves and structures.

    Most inherited endocrine conditions are rare and are usually related to deficient or dysfunctional production of a single hormone or to the hormone production of a particular gland (for example, congenital hypothyroidism). However, there are genetic conditions that affect multiple glands. Two that have been identified as affecting several endocrine glands are MEN-1 and MEN-2 (Multiple Endocrine Neoplasia, types 1 and 2). These conditions are related to alterations in specific genes, and they increase the lifetime risk that affected people will develop tumors in one or more of their endocrine glands.

  • Summary of Endocrine Glands
    This table includes a listing of endocrine glands, including the hormones they produce and their function.
     
    Endocrine Gland Location/ Description Hormone(s) Produced Hormone Function
    Hypothalamus Lower middle of the brain; communicates with both nervous and endocrine systems
    Growth hormone-releasing hormone (GHRH) Stimulates growth hormone production by the pituitary
    Thyrotropin-releasing hormone (TRH) Stimulates TSH production in the pituitary
    Corticotropin-releasing hormone (CRH) Stimulates ACTH production by the pituitary
    Gonadotropin-releasing hormone (GnRH) Stimulates LH and FSH production by the pituitary
    Prolactin inhibitory hormone (PIH, dopamine) Inhibits prolactin production
    Oxytocin; produced by the hypothalamus; stored and secreted by the pituitary Uterine contraction during labor
    Arginine vasopressin (AVP), also called antidiurectic hormone (ADH); produced by the hypothalamus; stored and secreted by the pituitary Water balance
    Somatostatin Inhibits growth hormone release from pituitary and also may have some effects on TSH and ACTH release
    Pituitary Below hypothalamus, behind sinus cavity Prolactin Milk production (milk production not related to pregnancy is called galactorrhea and is usually due to high prolactin)
    Growth hormone (GH) Stimulates childhood growth, cell production, helps maintain muscle and bone mass in adults
    ACTH Stimulates hormone  production (cortisol, androgens and, to a smaller degree, aldosterone) by the adrenal glands
    TSH Stimulates thyroid hormone production
    LH, FSH Regulation of testosterone and estrogen, fertility
    Thyroid Butterfly-shaped; lies flat against windpipe in the throat T4 (thyroxine)

    T3 (triiodothyronine)
    Help regulate the rate of metabolism
    Calcitonin         Helps regulate bone status, blood calcium
    Parathyroid 4 tiny glands located behind, next to, or below the thyroid Parathyroid hormone (PTH)        Regulates blood calcium
    Adrenal    2 triangular organs, on top of each kidney Epinephrine (adrenaline)
    Norepinephrine (Catecholamines)
    Blood pressure regulation, stress reaction, heart rate
    Aldosterone    Salt, water balance
    Cortisol Stress reaction
    DHEA-S Body hair development at puberty
    Ovaries (females only) 2, located in the pelvis Estrogen

    Progesterone
    Female sexual characteristics
    Testicles (males only) 2, located in the groin Testosterone Male sexual characteristics
    Endocrine Pancreas Large, gourd-shaped gland, located behind the stomach Insulin
    Glucagon
    Somatostatin

    Glucose regulation

    *In the GI tract, somatostatin inhibits the release of GI hormones such as secretin and gastrin.   

    Pineal Lower side of the brain Melatonin Not well understood;
    Helps control sleep patterns, affects reproduction
  • Endocrine Syndromes and Disorders

    Examples of endocrine disorders on this site:

    Click on the links below to read more about the disorders and related laboratory tests.

     

    Examples of other endocrine disorders:

    Information on these and many others can be found at the Hormone Health Network's article on Conditions and Diseases and in the MedlinePlus Medical Encyclopedia article on Endocrine Diseases. Also check the Related Content section of this article.

    • Acromegaly
    • C-cell hyperplasia
    • Diabetes insipidus
    • Multiple endocrine neoplasia type 2 (MEN 2)
    • Pheochromocytomas
    • Secondary hyperaldosteronism
    • Growth hormone deficiency
    • Zollinger-Ellison syndrome
    • Disorders of the timing and development of puberty (e.g., precocious puberty)
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