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U N I T 9
Endocrine System
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.
Primary, Secondary, andTertiary
Disorders
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.
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
■■
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.
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.
