What is the normal serum cortisol range?

Normal Serum Cortisol Range

Morning serum cortisol values greater than 300 nmol/L (10 μg/dL) indicate normal adrenal function, while values less than 150 nmol/L (5 μg/dL) suggest adrenal insufficiency. 1

Context-Dependent Interpretation

The interpretation of "normal" cortisol depends critically on timing of collection and clinical context:

Morning Cortisol (8:00-9:00 AM)

• >300 nmol/L (>10 μg/dL): Indicates adequate HPA axis function 1
• 150-300 nmol/L (5-10 μg/dL): Intermediate zone; HPA axis recovery possible, may require dynamic testing 1
• <150 nmol/L (<5 μg/dL): Very likely persistent adrenal insufficiency 1

More stringent thresholds exist in specific contexts:

• Morning cortisol ≥354 nmol/L (12.8 μg/dL) predicted normal adrenal function with 100% sensitivity in patients with suspected adrenal insufficiency 1
• Morning cortisol ≥276 nmol/L (10.0 μg/dL) was associated with no glucocorticoid withdrawal symptoms in patients recovering from Cushing syndrome 1

Traditional Reference Ranges

Historical data from healthy populations show:

• Morning specimens: 175 ± 53 μg/L (mean ± SD) 2
• Afternoon specimens: 103 ± 40 μg/L 2
• Nigerian population study: 72.0-554.0 nmol/L 3

Critical Factors Affecting Interpretation

Assay Variability

The specific cortisol assay used dramatically impacts reference ranges. 1 In one large study, baseline cortisol values that excluded adrenal insufficiency varied between 336 nmol/L (12.2 μg/dL) and 506 nmol/L (18.3 μg/dL) when measured by three different immunoassays 1.

You must know your laboratory's specific cutoffs rather than relying on universal values. Liquid chromatography-tandem mass spectrometry is most accurate, followed by monoclonal antibody immunoassays, with polyclonal antibody immunoassays being least specific 1.

Physiologic Variables That Alter Results

Falsely elevated cortisol:

• Pregnancy 1
• Oral estrogen use 1
• Elevated cortisol-binding globulin 1

Falsely decreased cortisol:

• Hypoalbuminemia (cirrhosis, nephrotic syndrome, malnutrition) 1
• Low cortisol-binding globulin 1
• Prolonged critical illness 1
• Disrupted circadian rhythm (night shift workers, jet lag, severe insomnia) 1

Timing Requirements

Cortisol secretion peaks just before waking, making 8:00-9:00 AM the standard collection time for morning cortisol 1. The test should be performed at least 24 hours after the last glucocorticoid dose (excluding dexamethasone) 1.

Age-Related Considerations

Pediatric populations have different reference ranges. Morning serum basal cortisol increases with age and pubertal maturation, with reference values of 2.97 μg/dL (81.9 nmol/L) at the 2.5th percentile and 23.4 μg/dL (645.5 nmol/L) at the 97.5th percentile in children aged 4-19 years 4.

Clinical Application Algorithm

For suspected adrenal insufficiency:

1. Measure morning cortisol (8:00-9:00 AM, fasting)
2. If <150 nmol/L (<5 μg/dL): Adrenal insufficiency very likely; initiate replacement
3. If 150-300 nmol/L (5-10 μg/dL): Consider dynamic testing (ACTH stimulation test)
4. If >300 nmol/L (>10 μg/dL): Normal HPA axis function; adrenal insufficiency unlikely

Common pitfall: Do not use a single arbitrary cutoff. The 2024 European Society of Endocrinology/Endocrine Society joint guideline emphasizes that cortisol values should be considered as a continuum rather than rigid thresholds, with higher values progressively more indicative of normal function 1.