Normal Cortisol Levels
Normal cortisol levels vary by specimen type and time of day: serum cortisol ranges from approximately 150-550 nmol/L (5-20 μg/dL) in the morning, salivary cortisol from 3.5-27 nmol/L in the morning and <6 nmol/L at night, and 24-hour urinary free cortisol should be measured with at least 2-3 collections to account for variability.
Serum Cortisol Reference Ranges
Morning serum cortisol levels demonstrate significant circadian variation, with peak values occurring shortly after awakening. Based on current guidelines and recent research:
Morning (8 AM) values: The most recent guideline evidence indicates that morning cortisol >300 nmol/L (10 μg/dL) suggests normal HPA axis function 1. More specifically, values ≥354 nmol/L (12.8 μg/dL) predict normal adrenal function with 100% sensitivity 1.
Lower threshold: Morning cortisol <150 nmol/L (5 μg/dL) strongly suggests adrenal insufficiency and warrants glucocorticoid replacement 1.
Intermediate zone: Values between 150-300 nmol/L require dynamic testing for definitive assessment 1.
A population-based study from 2024 established reference intervals of 72.0-554.0 nmol/L for serum cortisol, though this represents a broad range across the day 2. Research data consistently shows morning values around 459.6 ± 235.2 nmol/L in healthy individuals 3.
Critical Considerations for Serum Cortisol Interpretation
Assay-dependent variability is substantial. The specific immunoassay or mass spectrometry method used significantly affects reference ranges—baseline cortisol thresholds for excluding adrenal insufficiency varied from 336-506 nmol/L (12.2-18.3 μg/dL) across three different immunoassays 1. Clinicians must know their laboratory's specific reference ranges.
Binding protein effects alter total cortisol measurements:
- Elevated CBG (oral estrogens, pregnancy, chronic hepatitis) falsely increases total cortisol 4, 1
- Decreased CBG/albumin (nephrotic syndrome, cirrhosis, malnutrition, critical illness) falsely decreases total cortisol 4, 1
Circadian disruption (night shift work, jet lag, severe insomnia) invalidates morning cortisol interpretation 1.
Salivary Cortisol Reference Ranges
Salivary cortisol measures free (biologically active) cortisol and avoids binding protein confounders, making it particularly useful when CBG alterations are suspected.
Established reference ranges from guideline-cited research:
Additional research confirms morning values of 15.5 ± 0.8 nmol/L and evening values of 3.9 ± 0.2 nmol/L in healthy adults 6. Another study reported morning values of 21.2 ± 16.2 nmol/L and afternoon values of 12.7 ± 8.1 nmol/L 3.
Late Night Salivary Cortisol (LNSC)
LNSC is the most specific screening test for Cushing's syndrome 4. The diagnostic utility relies on loss of normal circadian nadir in hypercortisolism. Guidelines recommend:
- At least 2-3 LNSC measurements to account for variability 4
- Sampling at usual bedtime rather than strict midnight timing improves specificity 4
- Avoid in night-shift workers or those with disrupted sleep-wake cycles 4
Mass spectrometry versus immunoassay: Mass spectrometry offers better sensitivity but reduced specificity compared to immunoassay, and avoids contamination from topical hydrocortisone 4.
Urinary Free Cortisol (UFC)
At least 2-3 separate 24-hour urine collections are required to account for intra-patient variability 4. UFC measures overall cortisol production independent of CBG levels, providing an advantage over serum measurements in patients with binding protein abnormalities 4.
Research demonstrates strong correlation between 24-hour UFC and both serum (R=0.99) and salivary (R=0.97) cortisol 7. Mean 24-hour UFC in healthy individuals is approximately 170.8 ± 58.7 nmol/day 7.
Dynamic Testing Thresholds
When static cortisol values fall in indeterminate ranges, dynamic testing provides definitive assessment:
Dexamethasone Suppression Test (DST)
1 mg overnight DST: Normal suppression is cortisol <50 nmol/L (1.8 μg/dL) at 8 AM after 1 mg dexamethasone given at 11 PM 4. However, recent evidence suggests this threshold may be too low—a control-derived threshold of 66 nmol/L may be more appropriate 8.
Post-ACTH stimulation: Normal response is salivary cortisol >23.5 nmol/L at 60 minutes 6, or serum cortisol peak >180 μg/L (guideline threshold) or >127 μg/L (assay-specific studies) 9.
Common Pitfalls
False-positive DST results occur with:
- CYP3A4 inducers (phenobarbital, carbamazepine, St. John's wort) increasing dexamethasone metabolism 4
- Malabsorption or rapid gut transit 4
- Measuring dexamethasone levels concomitantly reduces false-positives 4
False-negative DST results occur with:
- CYP3A4 inhibitors (fluoxetine, cimetidine, diltiazem) decreasing dexamethasone metabolism 4