Part of “CHAPTER 74 – TESTS OF ADRENOCORTICAL FUNCTION“

The normal ranges of plasma cortisol and urinary free cortisol overlap the detection limit of most assays. In other words, a value of “zero” can be normal. Thus, reliable tests of adrenal insufficiency evaluate the ability of plasma cortisol to respond to an ACTH stimulus. The cosyntropin (Cortrosyn) test is the most convenient and reliable. This test is performed by administering an intravenous bolus of 250 μg of synthetic β1-24 ACTH and measuring the plasma cortisol concentration 45 and 60 minutes later.22 A plasma cortisol concentration of 20 μg/dL is the lower limit of the normal response. Anything less than this suggests impaired adrenal function.23 Localizing the cause of adrenal insufficiency to the adrenal gland itself or to the hypothalamic-pituitary unit can be accomplished efficiently by measuring plasma ACTH. High ACTH concentrations localize the defect to the adrenal gland; normal or low concentrations localize the defect to the hypothalamus or pituitary.24 The RIA for ACTH has been notoriously unreliable, but the commercially available two-site immunoradiometric assay measurements are of high quality.25