Morning Cortisol Timing for Adrenal Insufficiency Screening
A morning cortisol blood sample should be drawn between 8:00 AM and 9:00 AM when screening for adrenal insufficiency. 1
Rationale for the 8:00-9:00 AM Window
The 2024 joint European Society of Endocrinology and Endocrine Society clinical practice guideline explicitly recommends measuring morning serum cortisol between 8:00 and 9:00 AM for evaluating adrenal insufficiency 1. This timing is critical because:
- Cortisol secretion reaches its peak just minutes before waking up and follows a circadian rhythm 1
- The circadian rhythm ensures that cortisol levels are at their physiologic maximum during this window, making it the optimal time to detect deficiency
- Testing outside this window can produce falsely low results in individuals with disrupted circadian rhythms (night shift workers, jet lag, severe insomnia) 1
Key Testing Requirements
When ordering the morning cortisol:
- Hold glucocorticoid doses for at least 24 hours before testing (excluding dexamethasone) 1
- The test should only be performed after reaching physiologic equivalent daily doses (e.g., prednisone 4-6 mg daily or hydrocortisone 15-25 mg total daily) 1
- Ensure the sample is truly drawn in the 8:00-9:00 AM window—one study found that 60% of cortisol tests were uninterpretable because they were drawn outside the appropriate time window 2
Interpretation Thresholds
The guideline provides a continuum approach rather than rigid cutoffs 1:
- >300 nmol/L (10 μg/dL): Indicates HPA axis recovery; glucocorticoids can be safely stopped
- 150-300 nmol/L (5-10 μg/dL): Continue physiologic replacement and retest; consider dynamic testing if levels persist in this range
- <150 nmol/L (5 μg/dL): Very likely persistent adrenal insufficiency; continue replacement and retest after months
Common Pitfalls to Avoid
Timing errors are the most frequent mistake. Random cortisol measurements have no clinical value 2. The test must be performed in the morning window to be interpretable.
Factors that affect results:
- Elevated cortisol-binding globulin (pregnancy, oral estrogens) can falsely elevate results 1
- Low albumin states (cirrhosis, nephrotic syndrome, malnutrition) can falsely lower results 1
- Assay variability exists between different cortisol immunoassays—familiarize yourself with your laboratory's specific cutoff values 1
Clinical context matters: Even with biochemical suppression, the risk of clinically meaningful adrenal crisis remains very low in glucocorticoid-induced adrenal insufficiency 1. The test is sensitive but not specific, serving as a safeguard rather than an absolute requirement for continued tapering.