Normal 24-Hour Urinary Free Cortisol Reference Range
The normal reference range for 24-hour urinary free cortisol (UFC) is sex-dependent and assay-specific, but generally the upper limit of normal is approximately 45-165 nmol/24h (16-60 μg/24h) for most modern assays, with males having higher upper limits than females.
Key Reference Values by Assay Method
The most recent high-quality evidence establishes the following sex-specific upper limits:
Mass Spectrometry (LC-MS/MS) - Most Accurate Method
- Males: 238 nmol/day (86.3 μg/day) 1
- Females: 147 nmol/day (53.3 μg/day) 1
- Volume-corrected: 89-91 nmol/L (32-33 μg/L) for both sexes 1
Alternative LC-MS/MS reference ranges from validated studies:
- Mixed sex upper limit: 45.66 μg/24h 2 or 165 nmol/24h 3
- Sex-separated 95th percentile: Males 152 nmol/24h, Females 141 nmol/24h 3
Immunoassay Methods (Less Specific)
- RIA Males: 294 nmol/24h 4
- RIA Females: 208 nmol/24h 4
- ECLIA Males: 379 nmol/24h 4
- ECLIA Females: 277 nmol/24h 4
Critical Clinical Context from Guidelines
At least 2-3 separate 24-hour urine collections should be obtained to account for significant intra-patient variability (up to 50%) 5. A single elevated result is insufficient for diagnosis.
Important Factors Affecting UFC Interpretation
Patient factors that influence UFC levels 5:
- Sex (most significant predictor - males have higher values)
- Body mass index (BMI)
- Age
- Urinary volume (very high >5L/24h or very low)
- Sodium intake
- Glomerular filtration rate
When UFC Testing May Be Unreliable
Avoid or use alternative tests (late-night salivary cortisol) in 5:
- Renal impairment (CrCl <60 mL/min) - GFR strongly predicts UFC
- Significant polyuria (>5 L/24h)
- Patients where accurate 24-hour collection is unlikely
Diagnostic Performance
For Cushing's syndrome screening, UFC has >90% sensitivity but lower specificity compared to other first-line tests 5. The guideline notes that late-night salivary cortisol is the most specific test, while UFC and dexamethasone suppression test are least specific 5.
Values within 3-fold of the upper limit of normal may represent pseudo-Cushing's (non-neoplastic hypercortisolism from psychiatric disorders, alcohol use, obesity, PCOS) rather than true Cushing's syndrome 5.
Common Pitfalls
- Collection errors: Incomplete 24-hour collection is the most common cause of inaccurate results
- Assay-specific ranges: Always use the reference range specific to your laboratory's assay method
- Sex-specific interpretation: Failing to apply sex-specific cutoffs leads to false positives in males and false negatives in females
- Single measurement: Never diagnose based on one collection due to high day-to-day variability