A morning cortisol of 23.8 µg/dL is well above the normal threshold and indicates robust adrenal function in this 69-year-old woman.
Interpretation
This cortisol level definitively rules out adrenal insufficiency and requires no further testing. 1
The 2024 European Society of Endocrinology/Endocrine Society joint guideline establishes that morning serum cortisol >300 nmol/L (10 μg/dL) indicates recovery of HPA axis function 1. Your patient's value of 23.8 µg/dL (657 nmol/L) is more than double this threshold, placing her firmly in the normal range.
Evidence-Based Thresholds
The guidelines frame cortisol interpretation as a continuum rather than rigid cutoffs 1:
- <5 μg/dL (<150 nmol/L): Very likely adrenal insufficiency
- 5-10 μg/dL (150-300 nmol/L): Indeterminate; may need repeat testing or dynamic testing
- >10 μg/dL (>300 nmol/L): Indicates HPA axis recovery/normal function
- >12.8 μg/dL (>354 nmol/L): Predicted normal adrenal function with 100% sensitivity in one validation study 1
Research studies support even higher specificity at elevated levels. A 2021 study found that morning cortisol >444.7 nmol/L (>16.1 μg/dL) predicted normal response to insulin tolerance testing with 96.9% sensitivity 2. Multiple studies confirm that values >13-15 μg/dL reliably exclude adrenal insufficiency 3, 4, 5.
Clinical Context
Important caveats to consider:
- Timing matters: This should be a true morning sample (8:00-9:00 AM) when cortisol peaks 1
- Assay variability: Different cortisol assays can yield values varying by 30-50% 1. However, at 23.8 µg/dL, this patient is so far above any diagnostic threshold that assay differences are irrelevant
- Binding protein effects: Oral estrogens or pregnancy elevate cortisol-binding globulin and can falsely elevate total cortisol 1. If she's on oral estrogen replacement, consider this—though again, the value is high enough to remain reassuring even accounting for this
- Stress or illness: Acute stress, infection, or critical illness can transiently elevate cortisol 1. Ensure this was drawn in a non-stressed, ambulatory state
No Further Testing Needed
Do not proceed with ACTH stimulation testing or other dynamic tests. The guidelines explicitly recommend against routine dynamic testing when morning cortisol is this elevated 1. The 250 µg cosyntropin stimulation test is reserved for indeterminate values (5-10 μg/dL range) or when clinical suspicion remains high despite borderline results 6.
Bottom Line
At 23.8 µg/dL, this patient has normal—indeed, robust—adrenal function. If she has symptoms prompting this evaluation (fatigue, hypotension, etc.), look for alternative explanations. Her adrenal glands are working perfectly well.