In suspected adrenal insufficiency, a morning cortisol level above which value makes the diagnosis unlikely?

• A. 12 μg/dL
• B. 20 μg/dL
• C. 25 μg/dL
• D. 18 μg/dL

Answer: (D)

Morning cortisol level is used as a quick screening test to judge whether the adrenal axis is functioning adequately in suspected adrenal insufficiency. If the level in the morning is about 18 μg/dL or higher, the chance of true adrenal insufficiency is very low, so you can reasonably rule out the diagnosis without immediate stimulation testing. This threshold has a strong negative predictive value, meaning a result above it makes AI unlikely in most cases. If cortisol falls below this level, you can’t exclude AI and further testing, such as an ACTH stimulation test, is usually needed to assess adrenal reserve. So, a value around 18 μg/dL serves as the cut-off: values above it argue against AI, while lower values require additional evaluation.