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Cosyntropin Stimulation Test

The hypothalamus secretes corticotrophin releasing hormone (CRH), which stimulates the pituitary to secrete ACTH. ACTH then stimulates the adrenal gland to produce cortisol. Cortisol exerts negative feedback effects both at the pituitary and the hypothalamic levels. ACTH also contributes to skin pigmentation.

Cortisol deficiency can be caused by a deficiency of cortisol, ACTH, or CRH. Adrenal failure is known as Addison’s disease, while ACTH or CRH deficiency is described as central adrenal insufficiency.

Adrenal crisis is the clinical syndrome of hypotension, hypoglycemia, hyponatremia, acidosis and obtundation. The best way to detect cortisol deficiency is to measure a plasma cortisol level at the time of the crisis. Normally, the result should be 20 ug/dL or greater. Low results are consistent with cortisol deficiency.

In the absence of a crisis, the cosyntropin stimulation test can be used to diagnose adrenal insufficiency. Cosyntropin is a synthetic polypeptide consisting of the first 24 amino acids of ACTH. For this reason, this test is also known as the rapid ACTH test.

The cosyntropin stimulation test is performed in the following manner. The patient is instructed to fast overnight. The next morning, a blood sample is drawn to measure the basal plasma cortisol level. Then, 250 ug of cosyntropin is administered intramuscularly or intravenously. Plasma samples are then drawn at 30 and 60 minutes for cortisol measurement. If only a single sample is collected, it should be drawn at 45 minutes. In patients with normal adrenal function, baseline cortisol level should exceed 5 ug/dL. Following administration of cosyntropin, plasma cortisol levels should increase by at least 7 ug/dL above the basal level and peak at 20 ug/dL or greater. A normal response rules out primary disorders of the adrenal gland, but does not exclude partial ACTH deficiency with impaired reserve.

Subsequent measurement of plasma ACTH level, determines whether adrenal insufficiency is primary (Addison's disease) or secondary. Elevated ACTH levels indicate primary adrenal insufficiency while normal or low levels indicate secondary adrenal insufficiency.

Specimen requirement is one red top tube for each time period.

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