Normal Morning Cortisol Levels in Healthy Adults
In healthy adults, normal morning (8 AM) serum cortisol levels range from 5-23 μg/dL (138-635 nmol/L), with most values clustering between 10-20 μg/dL (276-552 nmol/L). 1
Reference Range Details
The optimal morning cortisol reference range has been well-established across multiple endocrine society guidelines:
- Morning cortisol levels >14 μg/dL (>386 nmol/L) effectively rule out adrenal insufficiency, providing a clinically useful threshold for excluding hypocortisolism 1, 2
- The broad reference range of 5-23 μg/dL (138-635 nmol/L) accounts for normal physiologic variation among healthy individuals 1, 2
- Most healthy adults will have values concentrated in the 10-20 μg/dL (276-552 nmol/L) range during morning hours 1, 2
Timing Considerations for Optimal Interpretation
Morning cortisol measurement timing is critical for accurate interpretation:
- Samples should be collected between 8:00 AM and 12:00 PM to capture the physiologic peak of the circadian rhythm 3
- Cortisol follows a distinct circadian pattern, with highest levels in early morning and progressive decline throughout the day 4
- For afternoon samples (12 PM-6 PM), different reference ranges apply, with lower expected values 3
Clinical Context for Interpretation
Understanding when morning cortisol values become clinically significant:
- Values >300 nmol/L (10.9 μg/dL) almost completely exclude ACTH-cortisol insufficiency in unstressed patients 2, 5
- Values <275 nmol/L (<10 μg/dL) in morning samples warrant further evaluation with dynamic testing (cosyntropin stimulation test) to assess for adrenal insufficiency 3
- A single random morning cortisol <110 nmol/L (<4 μg/dL) strongly suggests adrenal insufficiency and requires immediate further evaluation 5
Critical Factors That Falsely Elevate Cortisol
Several common medications and conditions can artificially raise total cortisol measurements without true hypercortisolism:
- Oral contraceptives and estrogen-containing medications increase cortisol-binding globulin (CBG), dramatically elevating total cortisol while free cortisol remains normal 1
- Pregnancy increases CBG levels, leading to elevated total cortisol that does not reflect true hypercortisolism 1
- Topical hydrocortisone preparations can contaminate samples and cause falsely elevated results 1
- Acute physical or psychological stress, including strenuous exercise within 24-48 hours, can transiently elevate morning cortisol 1
Important Clinical Pitfall to Avoid
The most critical error is interpreting elevated total cortisol as pathologic without considering CBG status—always inquire about oral contraceptives, estrogen therapy, and pregnancy before pursuing extensive workup for hypercortisolism 1. This single oversight accounts for a substantial proportion of unnecessary diagnostic evaluations for suspected Cushing's syndrome.