Sample Type

Serum (from blood sample)

Fasting Required

✅ Recommended (8–12 hours fasting for consistency)

Ideal Timing

Between 7:00 AM and 9:00 AM (circadian peak)

Normal Range (8 AM)

5–25 µg/dL (varies slightly by lab)

Department

Endocrinology / Internal Medicine

Turnaround Time

1–2 working days

Suspected Addison’s disease

Detect low cortisol (primary adrenal insufficiency)

Suspected Cushing’s syndrome

Detect abnormally high morning cortisol

Chronic fatigue or unexplained weight loss

Rule out adrenal suppression

Hypopituitarism evaluation

Determine ACTH-driven cortisol deficiency

Monitoring of steroid withdrawal

Assess adrenal recovery

< 5 µg/dL

Possible adrenal insufficiency (Addison's, pituitary dysfunction)

5–15 µg/dL

Borderline/indeterminate – may require stimulation test

> 15–25 µg/dL

Generally normal

> 25 µg/dL

Suggestive of Cushing’s syndrome or stress

ACTH

To differentiate adrenal vs pituitary cause

ACTH Stimulation Test (Synacthen)

Gold standard to confirm adrenal insufficiency

4 PM Cortisol

To check circadian rhythm pattern

Dexamethasone Suppression Test

To rule out Cushing’s syndrome

Aldosterone / Renin Ratio

For adrenal gland function

Electrolytes (Na+, K+)

Sodium low and potassium high in Addison’s

Blood Glucose

Cortisol affects glucose metabolism

Test Timing

8:00 AM (peak cortisol secretion)

Main Use

Diagnose adrenal insufficiency or excess

Next Steps if Abnormal

ACTH test, Dexamethasone suppression, 24h cortisol

Paired With

ACTH, Aldosterone, Renin, Electrolytes, CRH test