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The relationship of the A.M. and P.M. readings are used to determine adrenal response to stress.

Use Establish the diagnosis of adrenocortical insufficiency, Addison's disease, adrenocortical hypersecretion, Cushing's syndrome. Malfunction of the organs in the hypothalamic - pituitary - adrenal cortex axis will result in alteration of the cortisol levels. First among adrenal function tests for most needs. Elevated levels are found in the newborn period.

Methodology Immunochemiluminometric assay (ICMA)

Additional Information Cortisol is the major adrenal glucocorticoid steroid hormone, and is normally under feedback control by pituitary ACTH and the hypothalamus. Elevated values are found in the newborn and also before death.

Causes of low cortisol include pituitary destruction or failure, with resultant loss of ACTH to stimulate the adrenal, and metabolic errors or destruction of the adrenal gland itself (adrenogenital syndromes, tuberculosis, histoplasmosis). The diagnosis of hypoadrenalism generally requires confirmation with ACTH stimulation, due to the circadian rhythms of cortisol and other factors. Causes of increased cortisol, which may present initially simply as loss of normal diurnal variation, include pituitary overproduction of ACTH, production of ACTH by a tumor (notably oat cell cancers), and adrenal adenomas.

Dexamethasone suppression test helps distinguish among causes of elevated cortisol.1 Dexamethasone is a synthetic steroid which will suppress ACTH secretion. Under normal circumstances decreased cortisol levels follow. Suppressibility of elevated cortisol shows that feedback regulation is intact, and usually rules out Cushing's syndrome. If there is no suppression over night after 1 mg of dexamethasone, higher doses and longer times may suppress ACTH production by a pituitary adenoma, but will not suppress an adrenal adenoma. Measurement of ACTH may also be informative. For the diagnosis of adrenocortical insufficiency, the cosyntropin test is recommended. The corticotropin-releasing hormone (CRH) stimulation test (1 g/kg bovine CRH by I.V. administration) works well as the standard high-dose dexamethasone suppression test in distinguishing pituitary Cushing's disease from ectopic ACTH secretion. The test is less time consuming and can be an outpatient test. Cortisol in ACTH response is measured at timed intervals. A positive response (four times baseline) occurs in pituitary Cushing's disease, and a negative response is seen in ectopic ACTH-secreting tumor. There is a 97% positive predictive value. Cushing's is excluded in only 70% if there is nonresponse, however when both tests (dexamethasone suppression and CRH stimulation) show no response, there is a 100% predictive value for ectopic ACTH-secreting tumor. The stimulation test should not be used in hypoadrenalism. For the diagnosis of Cushing's syndrome (hypercortisolism), the urine free cortisol is the test of choice.

The dexamethasone suppression test (DST) is abnormal in many psychiatric illnesses. The sensitivity of the DST in major depression is approximately 40% to 50%, but is higher (60% to 70%) in severe (especially psychotic) affective disorders. False-positive results occur with alcoholism and with stress (eg, hospitalization).