Porth's Essentials of Pathophysiology, 4e

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Endocrine System

U N I T 9

gland or the absence of an enzyme needed for hormone synthesis. The gland may be destroyed by a disruption in blood flow, infection, inflammation, autoimmune responses, or neoplastic growth. There may be a decline in function with aging, or the gland may atrophy as the result of drug therapy or for unknown reasons. Some endocrine-deficient states are associated with receptor defects: hormone receptors may be absent, the recep- tor binding of hormones may be defective, or the cel- lular responsiveness to the hormone may be impaired. It is suspected that in some cases a gland may produce a biologically inactive hormone or that an active hormone may be destroyed by circulating antibodies before it can exert its action. Hyperfunction usually is associated with excessive hor- mone production. This can result from excessive stimu- lation and hyperplasia of an endocrine gland or from a hormone-producing tumor. Hyperfunction symptoms may also be paraneoplastic , meaning they are caused by hormones secreted by tumors of nonendocrine tissue (see Chapter 7). In addition to excessive endogenous hormone production, syndromes of hormone excess may result from deliberate or inadvertent administration of exoge- nous hormones. For example, administration of glucocor- ticoid drugs to suppress the inflammatory response may lead to Cushing syndrome, or androgen excess with sup- pression of pituitary gonadotropins may occur in athletes who take androgens to improve their performance. Endocrine disorders in general can be divided into primary, secondary, and tertiary disorders. Primary disorders originate in the target gland responsible for producing the hormone. In secondary disorders of endo- crine function, the target gland is essentially normal, but its function is altered by defective levels of stimulating hormones or releasing hormones from the pituitary gland. For example, total thyroidectomy produces a primary deficiency of thyroid hormones. Removal or destruction of the pituitary gland eliminates thyroid- stimulating hormone stimulation of the thyroid and brings about a secondary deficiency. A tertiary disorder results from hypothalamic dysfunction (as may occur with craniopharyngiomas or cerebral irradiation); thus, both the pituitary and target glands are understimulated. Primary, Secondary, andTertiary Disorders

■■ These disorders can occur as a primary defect in hormone production by a target gland that produces the hormone or as a secondary or tertiary disorder resulting from a defect in the hypothalamic-pituitary system that controls the target gland’s function.

Anterior Pituitary and Growth Hormone Disorders The pituitary gland, also known as the hypophysis, is a pea-sized gland located at the base of the brain, where it lies in a saddle-shaped depression in the sphenoid bone called the sella turcica. A short funnel-shaped stalk, the infundibulum, connects the pituitary gland with the hypothalamus. The pituitary gland has two com- ponents: a posterior lobe (neurohypophysis) or neural component, which stores and releases the oxytocin and the antidiuretic hormone (discussed in Chapter 8), and an anterior lobe (adenohypophysis) or glandular com- ponent (see Chapter 31, Fig. 31-3). Control of Anterior Pituitary Function The anterior lobe of the pituitary gland produces adre- nocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), growth hormone (GH), the gonado- tropic hormones (follicle-stimulating hormone [FSH] and luteinizing hormone [LH]), and prolactin. 2,3 The release of these hormones is in turn under the control of hor- mones produced in the hypothalamus (see Chapter 31). All of the anterior pituitary hormones, except for GH, exert their primary effects by controlling the biosynthesis and secretion of hormones from other target endocrine glands. ACTH controls the release of cortisol from the adrenal gland, TSH controls the secretion of thyroid hor- mone from the thyroid gland, and LH and FSH control sex hormone production and fertility. Clinical Manifestations of Hypothalamic- Pituitary Disorders Diseases of the pituitary are uncommon but may pres- ent with a variety of manifestations including pituitary hormone hypersecretion, hyposecretion, and/or the localized mass effect that causes compression of the optic chiasm or basal portion of the brain. In adults, the most common cause of hypothalamic-pituitary dys- function is a pituitary adenoma. In children, pituitary adenomas are uncommon; the most common cause of pituitary dysfunction is hypothalamic tumors, of which craniopharyngiomas (neoplasms arising from cells of an embryonic hypophysial structure) are the most common. These tumors usually manifest with signs of pituitary hyposecretion (e.g., short stature due to low GH levels and delayed puberty due to gonadotropin

SUMMARY CONCEPTS

■■ Disorders of the endocrine system, which provides the chemical messengers that serve to integrate the many functions of the body, can result from either a hypofunction or hyperfunction of an endocrine gland.