What is the confirmatory test for suspected adrenal insufficiency?

• A. Short ACTH (cosyntropin) stimulation test
• B. Dexamethasone suppression test
• C. Insulin tolerance test
• D. 24-hour urinary free cortisol

Answer: (A)

Assessing whether the adrenal cortex can respond to ACTH is the key idea. The short ACTH (cosyntropin) stimulation test directly tests the adrenal glands’ capacity to produce cortisol when stimulated. You measure baseline cortisol, give cosyntropin, and then recheck cortisol about 30–60 minutes later. A normal response means the peak cortisol meets or exceeds a threshold (lab-dependent, commonly around 18–20 μg/dL). An inadequate rise confirms adrenal insufficiency, reflecting failure of cortisol production by the adrenal cortex, whether due to primary gland destruction or secondary ACTH deficiency. Dexamethasone suppression tests are for Cushing’s syndrome, not AI. The insulin tolerance test can evaluate the HPA axis but is more complex and riskier, often not the first-line confirmatory test for AI. 24-hour urinary free cortisol screens for excess cortisol production, not deficiency.