Thyroid & Endocrine Fasting RequiredTurnaround: 12–24 hours

Serum Cortisol (Morning 8 AM & Afternoon / Evening)

Also known as: Total Serum Cortisol, Adrenal Stress Hormone, Hydrocortisone Test, Morning Cortisol, Diurnal Cortisol

Parameters Measured in the Report

A breakdown of individual biomarkers and biological reference intervals typically found in your report:

Biomarker	Standard Range	Unit	What It Means
Morning Cortisol (8:00 AM) (8 AM Cortisol)	6.0–23.0 (Peak diurnal concentration)	µg/dL (or 166–635 nmol/L)	Primary diagnostic window for adrenal function; physiological peak occurs shortly after waking.
Afternoon / Evening Cortisol (4:00 PM) (4 PM Cortisol)	3.0–15.0 (Trough concentration)	µg/dL	Should decline to roughly half of morning peak; failure to drop indicates loss of diurnal rhythm.

* Note: Reference ranges may vary slightly between testing instruments and reagent kits. Always refer to the reference interval printed on your specific pathology report.

Clinical Focus

Total circulating glucocorticoid steroid hormone synthesized by the adrenal cortex under pituitary ACTH regulation.

Sample & Specimen

Venous blood (Serum tube)

Fasting Protocol

Fasting from midnight preferred. Blood collection must occur precisely at 8:00 AM for morning peak.

Turnaround Time

12–24 hours

What Is This Test?

Serum Cortisol is the body’s primary glucocorticoid stress hormone, manufactured in the adrenal cortex located atop each kidney. Cortisol regulates carbohydrate and lipid metabolism, suppresses immune inflammation, maintains blood pressure, and coordinates the biological response to physical trauma and emotional stress. Cortisol production follows a strict **circadian diurnal rhythm**: levels peak sharply in the early morning between 7:00 AM and 9:00 AM (to prepare your body for waking activities) and decline steadily throughout the day, reaching their lowest trough around midnight. Timing the blood draw to 8:00 AM is essential for clinical interpretation.

Why Did Your Doctor Order This Test?

Physicians routinely prescribe the Cortisol Test in the following clinical scenarios:

Suspected Adrenal Insufficiency (Addison’s disease): profound chronic fatigue, unexplained weight loss, low blood pressure (hypotension), salt cravings, and dark bronze skin pigmentation

Suspected Cushing’s Syndrome (cortisol excess): rapid central weight gain, "moon face", buffalo hump fat pad behind the neck, purple abdominal stretch marks (striae), and easy bruising

Investigation of refractory low blood pressure (adrenal crisis) during acute severe illness or sepsis

Monitoring patients undergoing long-term steroid therapy (prednisone) to assess hypothalamic-pituitary-adrenal (HPA) axis recovery

How to Prepare Before Your Test

Fasting Requirement: Fasting from midnight preferred. Blood collection must occur precisely at 8:00 AM for morning peak.

Morning blood draw must be scheduled strictly between 7:30 AM and 8:30 AM (ideally 8:00 AM).
Rest quietly for 20 to 30 minutes before blood collection — acute physical exertion or anxiety causes temporary cortisol spikes.
Inform your physician if you take oral contraceptives, hormone replacement therapy, or steroid inhalers/creams.

Understanding Your Results

What higher or lower readings generally mean in clinical practice:

If Values Are Higher Than Normal

An abnormally elevated morning cortisol (>23–30 µg/dL) or failure of cortisol to suppress following an overnight Dexamethasone Suppression Test points toward Cushing’s syndrome (caused by an ACTH-secreting pituitary adenoma, adrenal cortical tumor, or ectopic ACTH secretion).

If Values Are Lower Than Normal

A low morning cortisol (<3.0–5.0 µg/dL) strongly suggests Adrenal Insufficiency (Addison’s disease or secondary pituitary failure). Patients with low morning cortisol require a confirmatory ACTH (Synacthen) Stimulation Test.

Frequently Asked Questions

Why is the exact time of blood collection so critical for cortisol?

Cortisol levels naturally fluctuate by over 50% between morning and night. An 8:00 AM level of 18 µg/dL is completely normal, but that exact same level of 18 µg/dL at 11:00 PM is abnormal and indicates Cushing’s syndrome.

How do birth control pills affect cortisol test results?

Estrogen in oral contraceptive pills increases Corticosteroid-Binding Globulin (CBG) in the blood. This falsely elevates measured Total Cortisol even though active free cortisol is completely normal.

Related Laboratory Tests

Tests frequently prescribed together or used as follow-up investigations:

Thyroid Profile (TFT: TSH, Free T3, Free T4)

Thyroid Stimulating Hormone (TSH), Free Thyroxine (FT4), and Free Triiodothyronine (FT3).

Serum Electrolytes (Sodium, Potassium, Chloride, Bicarbonate)

Electrically charged mineral ions: Sodium (Na+), Potassium (K+), Chloride (Cl-), and Bicarbonate (HCO3-).

Fasting Blood Sugar (FBS) & PPBS

Immediate concentration of glucose in the bloodstream after an overnight fast and post-meal.

Complete Blood Count (CBC / Hemogram)

Counts and evaluates red blood cells, white blood cells, hemoglobin, hematocrit, and platelets.

Medical & Diagnostic Disclaimer

This guide is published for educational and patient informational purposes only. It is not a substitute for professional medical evaluation, diagnosis, or clinical advice.

A laboratory result that falls slightly outside standard statistical reference intervals does not automatically indicate disease; conversely, a normal result does not guarantee the absence of medical conditions. Only your attending physician can interpret lab findings in the context of your medical history, physical exam, and symptoms.