What is the pituitary gland?

Structure and Functions

The pituitary gland is similar in size to a pea and has two lobes, the anterior (adenohypophysis) and the posterior (neurohypophysis). The anterior lobe accounts for a greater proportion of the total weight, approximately 80 percent. The pituitary gland is involved in the release of numerous hormones that have a multitude of effects throughout the body; as a result, it is often referred to as the “master” gland. The functions of the hormones released from the pituitary gland include reproductive functions (including childbirth and lactation), bone growth and development, and regulation of metabolic processes, body temperature, water balance, circulation, and blood pressure. Therefore, a normally functioning pituitary gland is essential to the health and maintenance of homeostasis in humans.

Hormones released from the anterior lobe of the pituitary gland include thyroid-stimulating hormone (TSH) or thyrotropin, growth hormone (GH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and prolactin (PRL). The hypothalamus, which is located just superior to the pituitary gland, regulates the release of hormones from the anterior pituitary gland by releasing hormones that travel through a blood network called the hypophyseal portal system to the anterior pituitary. For example, the hypothalamus releases growth-hormone-releasing hormone
(GHRH), which travels to the anterior pituitary and stimulates the release of growth hormone. Alternatively, if the body wants to decrease the secretion of GH, the hypothalamus may do so by releasing growth-hormone-inhibiting hormone (GHIH), also known as growth-hormone-release-inhibiting hormone (GHRIH) or somatostatin. Another common inhibiting hormone released from the hypothalamus is prolactin-inhibiting hormone (PIH), which reduces the release of PRL from the anterior pituitary.

In contrast to the anterior lobe, the posterior lobe of the pituitary gland releases two hormones: oxytocin and antidiuretic hormone (ADH), which is commonly referred to as vasopressin. The release of these hormones is influenced by blood pressure, osmolarity of the blood, and other inputs, such as those from the nervous and reproductive systems.

Several hormones released by the pituitary gland stimulate other organs or tissues directly, resulting in changes in overall physiologic function. For example, GH increases protein synthesis and the related growth of muscles, bones, and tissues. GH also increases the release of glucose and fat breakdown to fuel these anabolic processes. PRL stimulates the mammary glands and causes lactation, and oxytocin also stimulates lactation and causes contraction of the uterine wall during labor. Finally, ADH causes the kidneys to reabsorb or retain water, aiding in the body’s ability to regulate water balance and hydration levels.

Other hormones released by the pituitary gland stimulate the release of still other hormones from subsequent endocrine glands. These include TSH, which stimulates the thyroid gland to release thyroid hormones; ACTH, which stimulates the cortex of the adrenal glands to release cortisol; and the sex hormones FSH and LH, also called gonadotrophs, which stimulate the sex organs in males and females to release hormones involved in the production of sperm and the function of the menstrual cycle, respectively.

The release of most hormones from the pituitary gland is controlled by classic negative-feedback loops. In these systems, an increase in product feeds back to inhibit further stimulation of that system. Using the thyroid hormones as an example, the hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to release TSH, which then causes the thyroid gland to release thyroid hormones. When circulating levels of thyroid hormones are higher than normal or necessary, they feed back to inhibit or slow the release of TRH from the hypothalamus and TSH from the anterior pituitary. Conversely, when circulating levels of thyroid hormones are low, they feed back to increase the release of TRH and TSH, which will ultimately increase the release of thyroid hormones from the thyroid gland. Therefore, there are several sites of control involved in the release of products from the pituitary gland.

A notable exception to this trend is the presence of a positive-feedback process involving the pituitary gland and the hormone oxytocin. In this system, the stretching of the cervix during labor causes the release of oxytocin, which increases contractions in the uterus to assist with the progression of labor and eventual childbirth. The oxytocin feeds back in a positive fashion by increasing the stretching of the cervix, which then leads to the release of even more oxytocin. This cycle continues until the child is born, at which time the stimulus of cervix stretch and the related oxytocin release both cease.

Disorders and Diseases

Pituitary dysfunction is typically characterized by an oversecretion or undersecretion of pituitary hormones. An overactive pituitary gland is an endocrine defect characterized by excessive growth in stature and mass, plus a variety of other symptoms, depending on which hormones are elevated. The increased hormone release from the pituitary gland is often attributed to a pituitary tumor. If this is the case, then the tumor may be treated with radiation therapy, surgical removal, or the use of an antagonist to decrease the release of pituitary hormones.

Individuals with underactive pituitary glands, or hypopituitarism, experience symptoms such as short stature, low body mass, infertility or reproductive difficulties (including the inability of women to lactate following childbirth), low energy levels, perpetual feeling of cold due to an inability to regulate body temperature, and fatigue. Hypopituitarism may also be caused by a pituitary tumor, as well as injury to or infection of the hypothalamus or pituitary gland. The treatment for hypopituitarism involves stimulating the release of hormones from the target organs or tissues, rather than stimulating the pituitary gland itself.

Bibliography

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Daniel, P. M. “Anatomy of the Hypothalamus and Pituitary Gland.” Journal of Clinical Pathology Supplement (Association of Clinical Pathologists) 7 (1976): 1–7.

Freeman, Susan, L. “The Anterior Pituitary.” In Endocrine Pathophysiology, edited by Catherine B. Niewoehner. 2d ed. Raleigh, N.C.: Hayes Barton Press, 2004.

Harris, G. W. “Neural Control of the Pituitary Gland: I. The Neurohypophysis.” British Medical Journal 2, no. 4731 (September 8, 1951): 559–564.

Harris, G. W. “Neural Control of the Pituitary Gland: II. The Adenohypophysis, with Special Reference to the Secretion of ACTH.” British Medical Journal 2, no. 4732 (September 15, 1951): 627–634.

Klibanski, Anne, and Nicholas Tritos, eds. "Pituitary Disorders." Hormone Health Network, May 2013.

"Pituitary Disorders." MedlinePlus, July 11, 2013.