Management of Patients With Pituitary Disorders
The pituitary gland, or the hypophysis, is a round structure about 1.27 cm (1⁄2 inch) in diameter located on the inferior aspect of the brain. It is divided into the anterior, intermediate, and pos-terior lobes.
Commonly referred to as the master gland, the pituitary secretes hormones that control the secretion of hormones by other en-docrine glands (Fig. 42-2). The pituitary itself is controlled by the hypothalamus, an adjacent area of the brain connected to the pi-tuitary by the pituitary stalk.
The important hormones secreted by the posterior lobe of the pi-tuitary gland are vasopressin (antidiuretic hormone [ADH]) and oxytocin. These hormones are synthesized in the hypothalamusand travel from the hypothalamus to the posterior pituitary gland for storage. Vasopressin controls the excretion of water by the kidney; its secretion is stimulated by an increase in the osmolal-ity of the blood or by a decrease in blood pressure. Oxytocin fa-cilitates milk ejection during lactation and increases the force of uterine contractions during labor and delivery. Oxytocin secre-tion is stimulated during pregnancy and at childbirth.
The major hormones of the anterior pituitary gland are follicle-stimulating hormone (FSH), luteinizing hormone (LH), pro-lactin, ACTH, thyroid-stimulating hormone (TSH), and growth hormone (also referred to as somatotropin). The secretion of these major hormones is controlled by releasing factors secreted by the hypothalamus. These releasing factors reach the anterior pituitary by way of the bloodstream in a special circulation called the pituitary portal blood system. Other hormones include melanocyte-stimulating hormone and beta-lipotropin; the func-tion of lipotropin is poorly understood.
The hormones released by the anterior pituitary enter the gen-eral circulation and are transported to their target organs. The main function of TSH, ACTH, FSH, and LH is the release of hormones from other endocrine glands. Prolactin acts on the breast to stimulate milk production. Growth hormone has wide-spread effects on many target tissues and is discussed later. Hor-mones that stimulate other organs and tissues are discussed in conjunction with their target organs.
Growth hormone is a protein hormone that increases protein synthesis in many tissues, increases the breakdown of fatty acids in adipose tissue, and increases the glucose level in the blood. These actions of growth hormone are essential for normal growth, although other hormones, such as thyroid hormone and insulin, are required as well. Stress, exercise, and low blood glu-cose levels increase the secretion of growth hormone. The half-life of growth hormone activity in the blood is 20 to 30 minutes; the hormone is largely inactivated in the liver.
Abnormalities of pituitary function are caused by oversecretion or undersecretion of any of the hormones produced or released by the gland. Abnormalities of the anterior and posterior portions of the gland may occur independently. Oversecretion (hypersecretion) most commonly involves ACTH or growth hormone and results in Cushing’s syndrome or acromegaly, respectively. Acromegaly, an excess of growth hormone in adults, results in bone and soft tis-sue deformities and enlargement of the viscera without an increase in height. In children, oversecretion of growth hormone results in gigantism, with a person reaching 7 or even 8 feet tall. Conversely, insufficient secretion of growth hormone during childhood results in generalized limited growth and dwarfism.
Undersecretion (hyposecretion) commonly involves all of the anterior pituitary hormones and is termed panhypopituitarism. In this condition, the thyroid gland, the adrenal cortex, and the go-nads atrophy (shrink) because of loss of the trophic-stimulating hormones.
The most common disorder related to posterior lobe dys-function is diabetes insipidus, a condition in which abnormally large volumes of dilute urine are excreted as a result of deficient production of vasopressin.
Hypofunction of the pituitary gland (hypopituitarism) can result from disease of the pituitary gland itself or of the hypothalamus, but the result is essentially the same. Hypopituitarism may result from destruction of the anterior lobe of the pituitary gland. Pan-hypopituitarism (Simmonds’ disease) is total absence of all pitu-itary secretions and is rare. Postpartum pituitary necrosis (Sheehan’s syndrome) is another uncommon cause of failure of the anterior pituitary. It is more likely to occur in women with severe blood loss, hypovolemia, and hypotension at the time of delivery.
Hypopituitarism is also a complication of radiation therapy to the head and neck area. The total destruction of the pituitary gland by trauma, tumor, or vascular lesion removes all stimuli that are normally received by the thyroid, the gonads, and the adrenal glands. The result is extreme weight loss, emaciation, at-rophy of all endocrine glands and organs, hair loss, impotence, amenorrhea, hypometabolism, and hypoglycemia. Coma and death occur if the missing hormones are not replaced.
Pituitary tumors are usually benign, although their location and effects on hormone production by target organs can cause life-threatening effects. Three principal types of pituitary tumors represent an overgrowth of (1) eosinophilic cells, (2) basophilic cells, or (3) chromophobic cells (ie, cells with no affinity for either eosinophilic or basophilic stains).
Eosinophilic tumors that develop early in life result in gigantism. The affected person may be more than 7 feet tall and large in all proportions, yet so weak and lethargic that he or she can hardly stand. If the disorder begins during adult life, the excessive skeletal growth occurs only in the feet, the hands, the superciliary ridge, the molar eminences, the nose, and the chin, giving rise to the clinical picture called acromegaly. Enlargement, however, involves all tis-sues and organs of the body. Many of these patients suffer from se-vere headaches and visual disturbances because the tumors exert pressure on the optic nerves (Sachse, 2001). Assessment of central vision and visual fields may indicate loss of color discrimination, diplopia (double vision), or blindness of a portion of a field of vision. Decalcification of the skeleton, muscular weakness, and en-docrine disturbances, similar to those occurring in patients with hyperthyroidism, also are associated with this type of tumor.
Basophilic tumors give rise to Cushing’s syndrome with fea-tures largely attributable to hyperadrenalism, including masculin-ization and amenorrhea in females, truncal obesity, hypertension, osteoporosis, and polycythemia.
Chromophobic tumors represent 90% of pituitary tumors. These tumors usually produce no hormones but destroy the rest of the pituitary gland, causing hypopituitarism. People with this disease are often obese and somnolent and exhibit fine, scanty hair, dry, soft skin, a pasty complexion, and small bones. They also experience headaches, loss of libido, and visual defects pro-gressing to blindness. Other signs and symptoms include poly-uria, polyphagia, a lowering of the basal metabolic rate, and a subnormal body temperature.
Diagnostic evaluation requires a careful history and physical ex-amination, including assessment of visual acuity and visual fields. Computed tomography (CT) and magnetic resonance imaging (MRI) are used to diagnose the presence and extent of pituitary tu-mors. Serum levels of pituitary hormones may be obtained along with measurements of hormones of target organs (eg, thyroid, adrenal) to assist in diagnosis if other information is inconclusive.
Surgical removal of the pituitary tumor through a transsphenoidal approach is the usual treatment. Stereotactic radiation therapy, which requires use of a neurosurgery-type stereotactic frame, may be used to deliver external-beam radiation therapy precisely to the pituitary tumor with minimal effect on normal tissue. Other treatments include conventional radiation ther-apy, bromocriptine (dopamine antagonist), and octreotide (syn-thetic analog of growth hormone). These medications inhibit the production or release of growth hormone and may bring about marked improvement of symptoms. Octreotide (Sandostatin) may also be used preoperatively to improve the patient’s clinical condi-tion and to shrink the tumor.
Hypophysectomy, or removal of the pituitary gland, may be per-formed to treat primary pituitary gland tumors. It is the treat-ment of choice in patients with Cushing’s syndrome due to excessive production of ACTH by a tumor of the pituitary gland. Hypophysectomy may also be performed on occasion as a pallia-tive measure to relieve bone pain secondary to metastasis of ma-lignant lesions of the breast and prostate.
Several approaches are used to remove or destroy the pituitary gland: surgical removal by transfrontal, subcranial, or oronasal– transsphenoidal approaches or irradiation or cryosurgery. Even if surgery succeeds at removing the tumor, many of the features or symptoms of acromegaly will be unaffected (Sachse, 2001).
The absence of the pituitary gland alters the function of many body systems. Menstruation ceases and infertility occurs after total or near-total ablation of the pituitary gland. Replacement therapy with corticosteroids and thyroid hormone is necessary; .
Diabetes insipidus is a disorder of the posterior lobe of the pitu-itary gland characterized by a deficiency of antidiuretic hormone (ADH), or vasopressin. Great thirst (polydipsia) and large vol-umes of dilute urine characterize the disorder. It may be sec-ondary to head trauma, brain tumor, or surgical ablation or irradiation of the pituitary gland. It may also occur with infec-tions of the central nervous system (meningitis, encephalitis, tu-berculosis) or tumors (eg, metastatic disease, lymphoma of the breast or lung). Another cause of diabetes insipidus is failure of the renal tubules to respond to ADH; this nephrogenic form may be related to hypokalemia, hypercalcemia, and a variety of med-ications (eg, lithium, demeclocycline [Declomycin]).
Without the action of ADH on the distal nephron of the kidney, an enormous daily output of very dilute, water-like urine with a specific gravity of 1.001 to 1.005 occurs. The urine contains no abnormal substances such as glucose and albumin. Because of the intense thirst, the patient tends to drink 2 to 20 liters of fluid daily and craves cold water. In the hereditary form of diabetes in-sipidus, the primary symptoms may begin at birth. In adults, the onset of diabetes insipidus may be abrupt or insidious.
The disease cannot be controlled by limiting fluid intake be-cause the high-volume loss of urine continues even without fluid replacement. Attempts to restrict fluids cause the patient to ex-perience an insatiable craving for fluid and to develop hyper-natremia and severe dehydration.
The fluid deprivation test is carried out by withholding fluids for 8 to 12 hours or until 3% to 5% of the body weight is lost. The patient is weighed frequently during the test. Plasma and urine osmolality studies are performed at the beginning and end of the test. The inability to increase the specific gravity and osmolality of the urine is characteristic of diabetes insipidus. The patient continues to excrete large volumes of urine with low specific grav-ity and experiences weight loss, rising serum osmolality, and ele-vated serum sodium levels. The patient’s condition needs to be monitored frequently during the test, and the test is terminated if tachycardia, excessive weight loss, or hypotension develops.
Other diagnostic procedures include concurrent measure-ments of plasma levels of ADH (vasopressin) and plasma and urine osmolality, a trial of desmopressin (synthetic vasopressin) therapy and intravenous infusion of hypertonic saline solution. When the diagnosis is confirmed and the cause is not obvious (eg, head injury), the patient is carefully assessed for tumors that may be causing the disorder.
The objectives of therapy are (1) to replace ADH (which is usu-ally a long-term therapeutic program), (2) to ensure adequate fluid replacement, and (3) to identify and correct the underlying intracranial pathology. Nephrogenic causes require different man-agement approaches.
Desmopressin (DDAVP), a synthetic vasopressin without the vascular effects of natural ADH, is particularly valuable because it has a longer duration of action and fewer adverse effects than other preparations previously used to treat the disease. It is ad-ministered intranasally; the patient sprays the solution into the nose through a flexible calibrated plastic tube. One or two admin-istrations daily or every 12 to 24 hours usually control the symp-toms (Tierney, McPhee, & Papadakis, 2001).
Another form of therapy is the intramuscular administration of ADH, or vasopressin tannate in oil, which is used when the in-tranasal route is not possible. It is administered every 24 to 96 hours. The vial of medication should be warmed or shaken vigorously before administration. The injection is administered in the evening so that maximum results are obtained during sleep. Abdominal cramps are a side effect of this medication. Rotation of injection sites is necessary to prevent lipodystrophy.
Clofibrate, a hypolipidemic agent, has been found to have an antidiuretic effect on patients with diabetes insipidus who have some residual hypothalamic vasopressin. Chlorpropamide (Dia-binese) and thiazide diuretics are also used in mild forms of the disease because they potentiate the action of vasopressin. The patient receiving chlorpropamide should be warned of the possi-bility of hypoglycemic reactions.
If the diabetes insipidus is renal in origin, the previously de-scribed treatments are ineffective. Thiazide diuretics, mild salt depletion, and prostaglandin inhibitors (ibuprofen, indomethacin, and aspirin) are used to treat the nephrogenic form of diabetes insipidus.
The patient with possible diabetes insipidus needs encourage-ment and support while undergoing studies for a possible cranial lesion. The nurse needs to inform the patient and family about follow-up care and emergency measures. The nurse also needs to provide specific verbal and written instructions, show the patient how to administer the medications, and observe return demon-strations as appropriate. The nurse also advises the patient to wear a medical identification bracelet and to carry medication and in-formation about this disorder at all times. Vasopressin must be administered with caution if the patient has coronary artery dis-ease because the medication causes vasoconstriction.
The syndrome of inappropriate antidiuretic hormone (SIADH)secretion includes excessive growth hormone (ADH) secretionfrom the pituitary gland even in the face of subnormal serum os-molality. Patients with this disorder cannot excrete a dilute urine. They retain fluids and develop a sodium deficiency known as dilu-tional hyponatremia. SIADH is often of nonendocrine origin; forinstance, the syndrome may occur in patients with bronchogenic carcinoma in which malignant lung cells synthesize and release ADH. SIADH has also occurred with severe pneumonia, pneu-mothorax, and other disorders of the lungs, in addition to malig-nant tumors that affect other organs (Terpstra & Terpstra, 2000).
Disorders of the central nervous system, such as head injury, brain surgery or tumor, and infection, are thought to produce SIADH by direct stimulation of the pituitary gland. Some medications (vincristine, phenothiazines, tricyclic antidepressants, thiazide diuretics, and others) and nicotine have been implicated in SIADH; they either directly stimulate the pituitary gland or increase the sensitivity of renal tubules to circulating ADH.
Eliminating the underlying cause, if possible, and restricting fluid intake are typical interventions for managing this syndrome. Because retained water is excreted slowly through the kidneys, the extracellular fluid volume contracts and the serum sodium concentration gradually increases toward normal. Diuretics (eg, furosemide [Lasix]) may be used along with fluid restriction if severe hyponatremia is present.
Close monitoring of fluid intake and output, daily weight, urine and blood chemistries, and neurologic status is indicated for the patient at risk for SIADH. Supportive measures and ex-planations of procedures and treatments assist the patient to deal with this disorder (Terpstra & Terpstra, 2000).