HYPOTHALAMUS AND
PITUITARY GLAND
The
HYPOTHALAMUS and the pituitary gland are closely related. The hypothalamus modulates the activity of the anterior portion of the pituitary through release or release-inhibitor
hormones, which travel through a specialized portal vascular system to the pituitary. These neurohormones regulate the release of such hormones as growth hormone (GH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), prolactin (PRL), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH).
Secretion of the neurohormones is triggered by neurotransmitters such as dopamine and acetylcholine and affected by such factors as stress, plasma levels of corticosteroid, time of day, and estrogen levels.
Decreased secretion of hypothalamic-releasing hormones results in decreased pituitary
Hormone secretion, a state called secondary hypopituitarism. It is usually caused by tumors, encephalitis, or inflammatory lesions.
Oversecretion of GH is usually caused by a pituitary
tumor. If oversecretion occurs before the closure of the epiphyses of the long bones (maturity), an individual can reach a height of 2.1 to 2.4 m (7 to 8 ft), a condition termed pituitary gigantism. If oversecretion occurs after the closure of the epiphyses (in adults), the resulting condition, called acromegaly, is characterized by overgrowth of the mandible (jaw); bone overgrowth and thickening of soft tissue, resulting in coarsening of the facial features; wide hands and broad fingers; and, often, soreness of joints. GH excess may induce signs of deficiency of other hormones, resulting in such disorders as hypogonadism accompanied by menstrual disturbance in women. Treatment consists of heavy particle irradiation or surgical removal of the tumor or lesion.
Cushing's disease is caused by an€TH-secreting tumor in the anterior pituitary that leads to excess secretion of steroid hormone, particularly cortisol, from the adrenal cortex. The syndrome is characterized by a "moon" face and high blood pressure. Severe protein wasting results in thin skin that is easily bruised and rupture of the subdermal tissue that produces purple streaks. Also observed are pronounced muscle weakness and a redistribution of fatÑcollecting in the abdominal wall, face, and upper backÑas well as a thinning of the extremities.
Hypersecretion of prolactin due to a pituitary tumor or secondary hypopituitarism (removal of hypothalamic influence) is associated with galactorrhea, amenorrhea, and infertility.
Decreased production of all pituitary hormones simultaneously is termed panhypopituitarism. GH deficiency leads to growth retardation, or dwarfism, most often appearing in childhood.
Loss of TSH results in hypothyroidism, evidenced by intolerance to cold, slow speech, and anemia. ACTH loss leads to secondary hypoadrenocorticismÑin particular, the loss of cortisol secretion. Symptoms include weakness, circulatory insufficiency, and low blood pressure. Loss of FSH and LH leads to amenorrhea, atrophy of the genitals, absence of libido, absence of sperm and ova production, and infertility. The absence of prolactin inhibits lactation. Treatment of panhypopituitarism consists of replacing the lost hormones; however, fertility cannot be restored in this manner.
The hypothalamus synthesizes antidiuretic hormone (ADH, vasopressin) and oxytocin, which are stored in the posterior pituitary and are released in response to stress and suckling. A deficiency in ADH, called diabetes insipidus, causes large volumes of water to be lost in urine. This state results from either tumors or disease in the nuclei of the hypothalamus, where ADH is produced, or unresponsiveness of the kidney tubules to ADH. If the cause centers in the hypothalamus, the disease can be corrected with animal or synthetic ADH. If untreated in newborn infants, it results in mental retardation or death by dehydration.
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