Which baseline morning cortisol value would prompt ACTH stimulation testing for suspected adrenal insufficiency?

• A. Between 5 and 10 µg/dL
• B. Above 18–20 µg/dL
• C. Below 3 µg/dL
• D. Exactly 15 µg/dL

Answer: (C)

A very low baseline morning cortisol strongly signals impaired cortisol production and is the situation that prompts testing to confirm adrenal insufficiency. The morning value reflects the natural diurnal peak, so if it falls below about 3 µg/dL, it means the adrenal glands may not be producing enough cortisol even before stimulation. In this scenario, an ACTH stimulation test (cosyntropin test) is indicated to confirm the diagnosis and appraise how completely the adrenals can respond. If the morning level is well above 18–20 µg/dL, adrenal insufficiency is unlikely and testing is generally not needed. Values in between are inconclusive and require the stimulation test to determine the adrenal response. Remember, the exact cutoffs can vary by lab, but the principle is clear: very low baseline cortisol triggers the confirmatory test to assess adrenal function.