Adrenal & stress
Cortisol (AM)
Glucocorticoid output of the HPA axis; diurnal sampling matters more than absolute value.
Sample
Serum, 8 AM (within 1 h of waking)
Fasting
Not strictly required
Clinical reference range
5 to 25 µg/dL
AM
Conv: 5–25 µg/dL
Biology
What it measures
ACTH-driven; pulsatile with marked diurnal rhythm (peak 6–8 AM, nadir near midnight).
Clinical use
Why we order it
Screen for adrenal insufficiency (AM <3 highly suggestive; <5 abnormal) or Cushing's (midnight salivary or 24 h urinary free cortisol).
Lens · Clinical
Interpretation by lens
Clinical interpretation
Borderline AM cortisol → cosyntropin (250 µg) stim test, expect rise to ≥18.
Differential
Causes of abnormal values
Causes of HIGH
- ↑Cushing's (endogenous or exogenous)
- ↑Stress/illness
- ↑Pregnancy (CBG-bound rises)
- ↑Estrogens
Causes of LOW
- ↓Primary or secondary adrenal insufficiency
- ↓Suppression from exogenous steroids
Pitfalls
Pre-analytic & interpretation traps
- !Random or PM cortisol is uninterpretable. Always 8 AM.
- !Patients on glucocorticoid replacement: hold dose >12 h before testing.
Related