Introduction
The pituitary is a pea-sized gland that is housed within a bony structure (sella turcica) at the base of the brain. The sella turcica protects the pituitary but allows very little room for expansion.
The pituitary controls the function of most other endocrine glands and is therefore sometimes called the master gland. In turn, the pituitary is controlled in large part by the hypothalamus, a region of the brain that lies just above the pituitary. By detecting the levels of hormones produced by glands under the pituitary's control (target glands), the hypothalamus or the pituitary can determine how much stimulation the target glands need.
 See the figure Pituitary: The Master Gland.
The pituitary has two distinct parts: the anterior (front) lobe, which accounts for 80% of the pituitary gland's weight, and the posterior (back) lobe. The lobes are connected to the hypothalamus by a stalk that contains blood vessels and nerve cell projections (nerve fibers, or axons). The hypothalamus controls the anterior lobe by releasing hormones through the connecting blood vessels; it controls the posterior lobe through nerve impulses.
The anterior lobe of the pituitary produces and releases (secretes) six main hormones: growth hormone, which regulates growth and physical development and has important effects on body shape by stimulating muscle formation and reducing fat tissue; thyroid-stimulating hormone, which stimulates the thyroid gland to produce thyroid hormones; corticotropin (also called adrenocorticotropic hormone or ACTH), which stimulates the adrenal glands to produce cortisol and other hormones; follicle-stimulating hormone and luteinizing hormone (the gonadotropins), which stimulate the testes to produce sperm, the ovaries to produce eggs, and the sex organs to produce sex hormones (testosterone and estrogen); and prolactin, which stimulates the mammary glands of the breast to produce milk.
The anterior lobe also produces hormones that cause the skin to darken (melanocyte-stimulating hormone) and that inhibit pain sensations and help control the immune system (endorphins).
The posterior lobe of the pituitary produces only two hormones: antidiuretic hormone and oxytocin. Antidiuretic hormone (also called vasopressin) regulates the amount of water excreted by the kidneys and is therefore important in maintaining water balance in the body (see Section 12, Chapter 158). Oxytocin causes the uterus to contract during childbirth and immediately after delivery to prevent excessive bleeding. Oxytocin also stimulates contractions of the milk ducts in the breast, which moves milk to the nipple (the let-down) in lactating women.
The hormones produced by the pituitary are not all produced continuously. Most are released in bursts every 1 to 3 hours, with alternating periods of activity and inactivity. Some of the hormones, such as corticotropin, which controls the adrenal glands; growth hormone, which controls growth; and prolactin, which controls milk production, follow a circadian rhythm: The levels rise and fall predictably during the day, usually peaking just before awakening and dropping to their lowest levels just before sleep. The levels of other hormones vary according to other factors. For example, in women, the levels of luteinizing hormone and follicle-stimulating hormone, which control reproductive functions, vary during the menstrual cycle.
The pituitary gland can malfunction in several ways, usually as a result of developing a noncancerous tumor (adenoma). The tumor may overproduce one or more pituitary hormones; it may press on the normal pituitary cells, causing underproduction of one or more pituitary hormones; or it may cause enlargement of the pituitary gland, with or without disturbing hormone production. Sometimes there is overproduction of one hormone by a pituitary tumor and underproduction of another at the same time due to pressure. Too little or too much of a pituitary hormone results in a wide variety of symptoms.
Doctors can diagnose pituitary gland malfunction using several tests. Imaging tests, such as a computed tomography (CT) or magnetic resonance imaging (MRI) scan, can show whether the pituitary has enlarged or shrunk and can usually determine whether a tumor exists in the gland.
Doctors can measure the levels of pituitary hormones, usually by a simple blood test. Doctors select which pituitary hormone levels they want to measure depending on the person's symptoms. Some pituitary hormones are not easy to measure because their levels vary greatly during the day and according to the body's needs; measuring a random blood sample does not provide useful information.
For some of those hormones, doctors give a substance that would normally affect hormone production; then they measure the level of the hormone. For example, if a doctor injects insulin, the levels of corticotropin, growth hormone, and prolactin should increase. Rather than measuring growth hormone levels directly, doctors often measure another hormone, insulin-like growth factor I (IGF-I). Growth hormone is produced in bursts and its levels quickly fall, but IGF-I levels reflect the overall daily production of growth hormone. For all of these reasons, interpreting the results of blood tests for pituitary hormones is complex.
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