Normal Range for Random Serum Cortisol
The interpretation of random cortisol levels depends heavily on timing of collection and clinical context, but generally values >300 nmol/L (>10 μg/dL) suggest adequate adrenal function, while values <140-150 nmol/L (<5 μg/dL) indicate likely adrenal insufficiency 1.
Key Thresholds Based on Timing
Morning Cortisol (8 AM - 12 PM)
- >300 nmol/L (>10 μg/dL): Indicates adequate HPA axis function 1
- <150 nmol/L (<5 μg/dL): Very likely indicates adrenal insufficiency 1
- 150-300 nmol/L (5-10 μg/dL): Gray zone requiring further evaluation 1
More refined morning thresholds from recent validation studies:
- ≥275 nmol/L: Excludes adrenal insufficiency with 96% sensitivity 2
- <275 nmol/L: Warrants dynamic testing 2
Afternoon Cortisol (12 PM - 6 PM)
- ≥250 nmol/L: Excludes adrenal insufficiency with 96% sensitivity in outpatients 2
- <250 nmol/L: Requires further evaluation 2
Critical Clinical Context
The "normal range" varies significantly based on several factors that must be considered:
Factors Affecting Interpretation
Protein binding states (alter total cortisol without changing free cortisol):
- Elevated CBG: Pregnancy, oral estrogen use → falsely elevated cortisol 1
- Decreased CBG/albumin: Cirrhosis, nephrotic syndrome, critical illness, malnutrition → falsely low cortisol 1
Circadian rhythm disruption:
- Night shift workers, jet lag, severe insomnia → unreliable morning values 1
Assay variability:
- Different immunoassays show significant variation (reference limits ranging 418-574 nmol/L for stimulated tests) 3
- Mass spectrometry is most accurate but less commonly available 1
Practical Algorithm for Random Cortisol Interpretation
Step 1: Assess Timing
- Morning sample (preferred): Use 275-300 nmol/L threshold
- Afternoon sample: Use 250 nmol/L threshold
- Random timing unknown: Use most conservative approach (300 nmol/L)
Step 2: Apply Thresholds
- >300 nmol/L: Adrenal insufficiency excluded in most cases; no further testing needed 1
- <150 nmol/L: Adrenal insufficiency highly likely; proceed to replacement therapy and monitor 1
- 150-300 nmol/L: Gray zone; proceed to dynamic testing (ACTH stimulation test) 1
Step 3: Consider Confounders
Before interpreting any value, verify:
- Not on oral estrogen (if yes, add ~100 nmol/L to thresholds)
- No severe hypoalbuminemia (if yes, consider free cortisol or salivary cortisol)
- Normal sleep-wake cycle (if disrupted, morning values unreliable)
Common Pitfalls
Do not use a single universal cutoff: The 2024 joint European/Endocrine Society guidelines emphasize cortisol as a continuum rather than arbitrary cutoffs 1. Context matters enormously.
Do not ignore assay-specific differences: A cortisol of 350 nmol/L on one assay may not equal 350 nmol/L on another 1, 3. Know your laboratory's specific reference ranges and validation data.
Do not test during acute illness: Critical illness alters cortisol metabolism and binding proteins, making interpretation unreliable 1.
Do not rely on random cortisol alone in high-risk patients: If clinical suspicion for adrenal insufficiency is strong (hypotension, hyponatremia, hyperkalemia), proceed directly to dynamic testing or empiric treatment regardless of random cortisol 4.
Evidence Quality Note
The most recent and authoritative guidance comes from the 2024 joint European Society of Endocrinology/Endocrine Society guideline on glucocorticoid-induced adrenal insufficiency 1, which provides the 300 nmol/L threshold. The 2016 Endocrine Society guideline on primary adrenal insufficiency 3 established the <140 nmol/L threshold for definite insufficiency. Recent validation studies 2 refined morning-specific cutoffs to 275 nmol/L with excellent sensitivity.