Confirming diagnosis A low-dose dexamethasone suppression test confirms the diagnosis of Cushing’s syndrome. Salivary cortisol levels collected at midnight (usually performed on an outpatient basis) are elevated and are the most sensitive confirmatory test. (See Diagnosing Cushing’s syndrome.)
A high-dose dexamethasone suppression test can determine if Cushing’s syndrome results from pituitary dysfunction (Cushing’s disease). In this test, dexamethasone suppresses plasma cortisol levels, and urinary 17-hydroxycorticosteroid (17-OHCS) and 17-ketogenic steroid levels fall to 50% or less of basal levels. Failure to suppress these levels indicates that the syndrome results from an adrenal tumor or a nonendocrine, corticotropin-secreting tumor. This test can produce false-positive results.
In a stimulation test, administration of metyrapone, which blocks cortisol production by the adrenal glands, tests the ability of the pituitary gland and the hypothalamus to detect and correct low levels of plasma cortisol by increasing corticotropin production. The patient with Cushing’s disease reacts to this stimulus by secreting an excess of plasma corticotropin as measured by levels of urinary 17-OHCS. If the patient has an adrenal or a nonendocrine corticotropin-secreting tumor, the pituitary gland — which is suppressed by the high cortisol levels — can’t respond normally, so steroid levels remain stable or fall.
Ultrasound, computed tomography (CT) scan, or angiography localizes adrenal tumors; CT scan and magnetic resonance imaging of the head may identify pituitary tumors.
» READ BOOK EXCERPT ONLINE »
Source: Professional Guide to Diseases (Eighth Edition), 2005
Cushing's syndrome:
Diagnosis
(Handbook of Diseases)
Initially, the diagnosis of Cushing’s syndrome requires determination of plasma steroid levels. In persons with normal hormone balance, plasma cortisol levels are higher in the morning and decrease gradually through the day (diurnal variation). In patients with Cushing’s syndrome, cortisol levels don’t fluctuate and typically remain consistently elevated; a 24-hour urine sample demonstrates elevated free cortisol levels.
Suppression test
A low-dose dexamethasone suppression test confirms the diagnosis of Cushing’s syndrome. A high-dose dexamethasone suppression test can determine whether Cushing’s syndrome results from pituitary dysfunction (Cushing’s disease) or from adenoma of the adrenal gland or ectopic corticotropin secretion.
In this test, dexamethasone suppresses plasma cortisol levels, and urinary 17-hydroxycorticosteroids (17-OHCS) and 17-ketogenic steroids fall to 50% or less of basal levels. Failure to suppress these levels indicates that the syndrome results from an adrenal tumor or a nonendocrine, corticotropin-secreting tumor. This test can produce false-positive results.
Stimulation test
In a stimulation test, administration of metyrapone, which blocks cortisol production by the adrenal glands, tests the ability of the pituitary gland and the hypothalamus to detect and correct low levels of plasma cortisol by increasing corticotropin production. The patient with Cushing’s syndrome reacts to this stimulus by secreting an excess of plasma corticotropin as measured by levels of urinary 17-OHCS.
If the patient has an adrenal or a nonendocrine corticotropin-secreting tumor, the pituitary gland—which is suppressed by the high cortisol levels — can’t respond normally, so steroid levels remain stable or fall. The corticotropin-releasing hormone tests can also be used to help differentiate pituitary Cushing’s and ectopic corticotropin syndrome.
Other tests
Ultrasonography, a computed tomography (CT) scan, magnetic resonance imaging (MRI), or angiography localizes adrenal tumors; a CT scan and MRI of the head identify pituitary tumors.
Clinical tip Measuring resting cortisol at midnight is the most sensitive test. This can be done at home by collecting saliva at midnight. In the hospital, collecting serum from an indwelling catheter placed in the early evening may aid collection.
» READ BOOK EXCERPT ONLINE »
Source: Handbook of Diseases, 2003
EXTREMITY, HAND, AND FOOT DEFORMITIES:
Approach to the Diagnosis
(Differential Diagnosis in Primary Care)
It is usually a simple matter to decide whether the deformity is due to
neurologic disease or to joint or bone disease. An x-ray film of the hands
or feet may be useful in acromegaly and many congenital disorders. Referral
to an orthopedic or neurologic specialist is usually indicated if bone or
neurologic involvement is probable. An arthritis workup can be done if joint disease is the
cause of the deformity.
» READ BOOK EXCERPT ONLINE »
Source: Differential Diagnosis in Primary Care, 2007
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