Normal Range for 2:00 PM Serum Cortisol
In healthy adults, afternoon serum cortisol (measured around 2:00 PM) typically ranges from approximately 3-16 μg/dL (83-441 nmol/L), though this represents a significant decline from morning peak levels due to normal circadian rhythm.
Understanding Cortisol's Circadian Pattern
Cortisol secretion follows a robust circadian rhythm, with the highest levels occurring in the early morning hours around awakening and progressively declining throughout the day 1. The majority of cortisol secretion occurs within several hours of morning awakening, with levels dropping substantially by afternoon 1.
Afternoon Cortisol Reference Values
While the provided guidelines focus primarily on morning cortisol measurements for diagnostic purposes, they offer important context for afternoon values:
Morning cortisol in healthy individuals typically exceeds 300 nmol/L (10 μg/dL), with values ≥354 nmol/L (12.8 μg/dL) predicting normal adrenal function with 100% sensitivity 2
Afternoon cortisol is expected to be significantly lower than morning values due to the natural circadian decline
Research validating single cortisol measurements suggests that afternoon samples (12 PM-6 PM) with values <250 nmol/L (9 μg/dL) warrant further investigation for adrenal insufficiency 3
Key Clinical Considerations
Assay variability matters significantly. Different cortisol assays can produce results varying by as much as 170 nmol/L (6.1 μg/dL) for the same sample 2. You must know your laboratory's specific reference ranges and methodology (immunoassay vs. mass spectrometry).
Factors that falsely elevate afternoon cortisol:
- Oral estrogen use or pregnancy (increases cortisol-binding globulin)
- Stress or acute illness
- Disrupted sleep-wake cycles 2
Factors that falsely lower afternoon cortisol:
- Hypoalbuminemic states (cirrhosis, nephrotic syndrome, malnutrition)
- Critical illness
- Low cortisol-binding globulin 2
Practical Interpretation
For a 2:00 PM cortisol measurement in a healthy adult:
- Values >10 μg/dL (>275 nmol/L) are generally reassuring for normal adrenal function
- Values 5-10 μg/dL (138-275 nmol/L) represent the typical afternoon range but require clinical context
- Values <5 μg/dL (<138 nmol/L) may warrant further evaluation if adrenal insufficiency is suspected, though this depends heavily on clinical presentation 3
The critical caveat: Afternoon cortisol is less reliable than morning cortisol for diagnostic purposes. If you're evaluating adrenal function, morning samples (ideally between 8 AM-12 PM) provide more clinically useful information 3. The afternoon nadir makes interpretation more challenging and increases the risk of false-positive results suggesting insufficiency in otherwise healthy individuals.