The adrenal glands are small,
paired structures located just above each kidney. The outer zone of the adrenal
gland (the cortex) is composed of glandular cells that manufacture and release
corticosteroids. There are two types of corticosteroids: mineralocorticoids and
Mineralocorticoids regulate electrolyte concentrations. Glucocorticoids reduce
inflammation and suppress the immune system. Glucocorticoids
are the type of corticosteroids used in nearly all steroid medications. The
output of corticosteroids from the adrenal glands is controlled by the
pituitary gland through the production of adrenocorticotropic hormone
Cushing’s syndrome is a disease caused by long-term exposure to high levels
of glucocorticosteroids, either manufactured by the body or given as
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Tumors of the pituitary gland that secrete ACTH stimulate the adrenal glands
to produce large amounts of adrenal hormones. This sustained overproduction in
response to pituitary stimulation accounts for 85 percent of cases of
spontaneous Cushing’s syndrome. The remaining 15 percent are caused by
corticosteroid-producing tumors of the adrenal glands themselves.
Spontaneous Cushing’s syndrome occurs primarily in middle-aged and older dogs, although dogs of all ages can be affected.
Poodles, Boston Terriers, Dachshunds, and Boxers have the highest
A number of cases of Cushing’s syndrome are caused by long-term therapy with
drugs that contain corticosteroids. This is called iatrogenic Cushing’s
Excess cortisone can cause hair loss over the body in a
symmetric pattern, with darkening of the underlying skin. The remaining hair is dry
and dull. Small blackheads may be found on the abdomen. The abdomen is
pendulous, distended, and pot-bellied. Other signs include lethargy with
reduced activity, infertility in females, testicular atrophy and infertility in
males, loss of muscle mass, and weakness. Excessive thirst and frequent
urination also occur.
Dogs with hyperadrenocorticism lose body condition and develop severe
problems such as hypertension, congestive heart failure, and
diabetes mellitus. Other complications include increased susceptibility to
infections, blood clots in the circulatory system (thromboembolism), and
central nervous system signs including behavioral changes and seizures.
The diagnosis of Cushing’s syndrome is based on laboratory tests, especially
those that measure serum cortisol concentrations before and after the injection
of ACTH and dexamethasone. Advances in CT scans and magnetic resonance imaging
(MRI)techniques have made it possible to visualize small tumors of the
pituitary and adrenal glands. Ultrasound can also be used to measure the size
and symmetry of the adrenal glands.