PITUITARY / HYPOTHALAMUS

The pituitary has been called the “master gland” because it controls several other hormone producing glands in the body. The pituitary is actually controlled by a small portion of the brain called the hypothalamus. The hypothalamus is located directly above the pituitary. The pituitary itself is located at the approximate intersection of an imaginary line drawn between the temples and another imaginary line drawn directly back from the bridge of the nose.

The pituitary gland has an anterior lobe, a posterior lobe and a stalk that connects it to the hypothalamus. The posterior lobe of the pituitary produces ADH (antidiuretic hormone) a hormone that controls the ability of the kidney to concentrate urine and maintain fluid and electrolyte balance in the body. A deficiency of ADH results in a condition called diabetes insipidus. A patient with this problem experiences constant thirst and constant urination. It can be treated and completely controlled with appropriate medication.

The anterior pituitary produces six hormones: growth hormone (hGH), prolactin, follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH) and adrenecorticopic stimulating hormone (ACTH). Each of these hormones is controlled by hormones from the hypothalamus.

The hormones FSH and LH are called gonadotropins because they stimulate the gonads (ovaries and testes). These hormones stimulate sex hormone production in both sexes and ovulation in women and sperm production in men. The thyroid gland is stimulated to produce thyroid hormone by TSH from the pituitary gland and the adrenal glands are stimulated to produce cortisol by ACTH. The hypothalamus stimulates FSH and LH secretion by release of GnRH (gonadotropin releasing hormone). It stimulates TSH secretion by releasing TRH (thyrotropin releasing hormone) and ACTH secretion by releasing CRH (corticotrophin releasing hormone). The releasing hormones are regulated by the level of hormone secreted by the target glands; sex hormones for GnRH, thyroid hormone for TRH and cortisol for CRH. Thus, there exists what is called “negative feedback” control of the hypothalamic/pituitary target gland axis. For example, when the level of thyroid hormone in the blood stream declines, the hypothalamus recognizes this and releases TRH to stimulate the pituitary to release TSH to stimulate the thyroid gland to produce more thyroid hormone. An inability of the target gland to produce hormone results in constant stimulation of the pituitary by the hypothalamus and a constant high level of the pituitary hormone. Growth hormone release is stimulated by GHRH (growth hormone releasing hormone) but no controlling negative feedback has been found for GHRH. Prolactin release is stimulated by TRH, the same hormone that stimulates TSH release.

Functionally, the gonadotropins control gonadal function, TSH controls thyroid function, ACTH controls adrenal function, growth hormone controls growth and prolactin controls lactation. The function of growth hormone in adults and prolactin in men and women that are not nursing is unclear.

Problems occur in the hypothalamic/pituitary system when either an excess or a deficiency of one or more of pituitary hormones appears. Excesses of pituitary hormones produce specific symptoms depending on which hormone is present in excess. These excesses are usually caused by pituitary tumors that produce these hormones. Deficiencies of pituitary hormones may result from destruction of part or all of the pituitary or hypothalamus by tumors or other space occupying lesions or by loss of blood supply to the area. Most endocrine problems related to the hypothalamic/pituitary function can be diagnosed and treated by specific laboratory and imaging studies.

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