What cortisol level is indicative of adrenal insufficiency after a 1-hour cosyntropin stimulation test?

In the context of evaluating adrenal insufficiency using the 1-hour cosyntropin stimulation test, a cortisol response level of ≥20 mg/dL is indicative of adequate adrenal function and suggests that adrenal insufficiency is less likely. During this test, cosyntropin—a synthetic form of ACTH—is administered to stimulate the adrenal glands to produce cortisol.

A baseline cortisol level is measured prior to administration, and a subsequent cortisol level is assessed after 1 hour. If the measured cortisol level rises to ≥20 mg/dL post-stimulation, it indicates that the adrenal glands are responding appropriately to the ACTH stimulus. An insufficient rise in cortisol levels, specifically if the post-stimulation measurement is below this threshold, would suggest adrenal insufficiency.

This cut-off reflects established clinical guidelines and understanding, where cortisol levels are expected to rise significantly if the adrenal glands are functioning normally. Therefore, a result of ≥20 mg/dL is a critical marker for distinguishing between normal adrenal function and adrenal insufficiency. Lower thresholds such as ≥10 or ≥15 mg/dL do not adequately capture the necessary hormone response required, while the option of ≥25 mg/dL is typically considered too high for initial evaluation.