War will never cease until babies begin to come into the world with larger cerebrums and smaller adrenal glands.
Henry Louis Mencken (US Journalist).
The endocrine system of the body comprises a group of glands and organs that regulate and
control various body functions by releasing specific proteins or steroids known as hormones. These are released into the bloodstream where they act as messengers, affecting the various activities of different parts of the body. When a particular hormone reaches its target cell it transmits its message by binding to a receptor, on the cell surface or in the cell nucleus, causing the cell to take a specific action. In this way, very small amounts of hormones can trigger very large responses in the body, controlling the function of entire organs, affecting such diverse processes as growth and development, reproduction, and sexual characteristics.
Although hormones circulate throughout the body, each type of hormone influences only certain organs and tissues. Some hormones affect only one or two organs, whereas others have influence throughout the body. The endocrine system comprises the hypothalamus, the pituitary gland, the thyroid gland, the parathyroid glands, the islets of the pancreas, the adrenal glands, the testes in men, and the ovaries in women. The secretion of each hormone must be regulated within precise limits, and so many endocrine glands are controlled by the interplay of hormonal signals between the hypothalamus, located in the brain, and the pituitary gland, which sits at the base of the brain.
The hypothalamus is a region of the brain that secretes a number of hormones known as releasing hormones, as these further stimulate the secretion of hormones from the pituitary gland. For example, gonadotropin-releasing hormone released from the hypothalamus signals the pituitary to produce follicle stimulating hormone and leutenising hormone which, in turn, stimulates the ovaries or testes to produce yet further hormones, such as testosterone and oestrogen, to signal sexual development. An inherited deficiency of gonadotropin-releasing hormone, therefore, results in the absence or decreased function of testes or ovaries, and is known as Kallmann syndrome.
Two further hypothalamic hormones are vasopressin and oxytocin which are released through the pituitary. A major function of vasopressin is to maintain water balance by regulating levels of water excreted by the kidneys. A lack of vasopressin leads to large amounts of water being excreted by the kidneys - a condition known as diabetes insipidus.
The pituitary (Lat. pituita; mucus, as this gland was once thought to produce nasal mucus) gland is a pea-sized structure located at the base of the brain. It secretes several hormones including growth hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, prolactin, melanocyte-stimulating hormone and endorphins that inhibit pain sensations and help control the immune system. Too much growth hormone occurring from early life or only in adulthood leads to gigantism and acromegaly, while growth hormone deficiency leads to proportional short stature. Too much prolactin, known as hyperprolactineamia, can trigger lactation and so is often associated with phantom pregnancy syndrome.
The thyroid gland is situated to the front of the neck and is named after an ancient Greek army shield, known as "thyreos", which is shaped like a door with a notch at the top for the soldier's chin. This gland secretes the hormone thyroxine that controls metabolism, respiration, and heart function. Mutations disrupting the function of thyroxine result in a disorder known as cretinism (though most cases of cretinism are the result of iodine deficiency) while an underactive thyroid gland present at birth, is known as congenital hypothyroidism while diseases such as Grave's disease lead to increased hormone production, known as hyperthyroidism.
The pair of adrenal glands (Lat. ad; above, renes; kidneys) are located just above the kidneys. These secrete two steroids, aldosterone, which acts to conserve sodium ions and water in the body, and cortisol which increases blood glucose levels. They also secrete two hormones, epinephrine and norepinephrine, particularly during stress. When the adrenal glands become underactive Addison's disease occurs, when they are overactive Cushing’s disease develops. Disruptions in the insulin hormone, produced by the pancreas lead to diabetes mellitus.
The testes control maleness is by producing male sex steroid hormones known as androgens (testosterone and dihydrotestosterone) that stimulate development of the male sex organs in addition to other male characteristics. Mutations in the androgen receptor lead to androgen insensitivity syndrome, while mutations in the gene for enzymes important for producing either dihydrotestosterone and oestrogen cause 5-alpha-reductase deficiency, aromatase deficiency or congenital adrenal hyperplasia.