Cortisol (AM) Blood Test

What Does This Test Measure?

Cortisol is the primary glucocorticoid hormone, produced by the adrenal cortex under ACTH stimulation from the pituitary. It follows a strong circadian rhythm, highest in the early morning (~6–8 AM), lowest around midnight. Cortisol regulates metabolism, immune response, blood pressure, and the stress response.

Why Your Doctor Ordered This Test

Doctors order a morning cortisol test when signs or symptoms point toward an adrenal disorder. In the case of suspected hypercortisolism, the classic picture includes central weight gain, purple stretch marks larger than 1 cm, easy bruising, a rounded face, and thinning skin. Less specific clues such as new or worsening hypertension, unexplained osteoporosis, and insulin resistance also raise suspicion. AM cortisol serves as a first-line screen: a markedly elevated morning value, particularly if the normal circadian dip is lost, suggests Cushing syndrome, though it must be verified with late-night salivary cortisol, 24‑hour urine free cortisol, or a low‑dose dexamethasone suppression test.

Suspected adrenal insufficiency prompts the test in patients with chronic fatigue, unintentional weight loss, low blood pressure, salt craving, and hyperpigmentation of gums or scars. A morning cortisol below 3 µg/dL is strongly suggestive of adrenal insufficiency, while values above 19 µg/dL generally rule it out. Intermediate results (5‑19 µg/dL) require an ACTH stimulation test to assess adrenal reserve. The test is also used to monitor the HPA axis recovery after long‑term glucocorticoid therapy and to investigate hypertension with hypokalemia, where it helps differentiate primary hyperaldosteronism from Cushing‑related mineralocorticoid excess.

What High Cortisol Means

High cortisol (hypercortisolism): Cushing syndrome, ACTH-dependent (pituitary adenoma = Cushing disease, ~70% of cases; ectopic ACTH from small-cell lung cancer or carcinoid) or ACTH-independent (adrenal adenoma or carcinoma, exogenous glucocorticoid use, most common cause overall). Pseudo-Cushing: severe depression, alcohol use disorder, poorly controlled diabetes, obesity, pregnancy, and severe stress (all can elevate cortisol without true Cushing syndrome). Diagnosis requires confirmation with late-night salivary cortisol, 24-hour urine free cortisol, or dexamethasone suppression test.

What Low Cortisol Means

Low cortisol: adrenal insufficiency, primary (Addison disease, autoimmune destruction of adrenal cortex; low cortisol + high ACTH; also causes hyperpigmentation, hyperkalemia, hyponatremia) or secondary (pituitary failure (low cortisol + low/normal ACTH; caused by prolonged glucocorticoid use with abrupt cessation) the most common cause of adrenal insufficiency worldwide, pituitary tumors, or pituitary surgery/radiation). Adrenal crisis (severe hypotension, shock) can occur with acute adrenal insufficiency or stress in patients with chronic insufficiency.

How to Prepare and What Affects the Result

Timing is the most critical factor. The blood draw must occur between 6 am and 8 am when the circadian peak is greatest. Strict fasting is rarely mandated, but some labs request no food or drink except water for 1‑2 hours before the draw; follow the specific lab instruction. For 24‑hour urine free cortisol, no special diet is needed, but excessive fluid intake should be avoided.

Physical and emotional stress can transiently elevate cortisol, so patients should avoid vigorous exercise, acute illness, or intense anxiety for several hours beforehand. Caffeine, alcohol, and smoking are discouraged on the morning of the test. Many medications distort results: oral contraceptives and estrogen replacement raise cortisol‑binding globulin and total cortisol, creating factitious hypercortisolism; oral, inhaled, or topical glucocorticoids suppress the axis; spironolactone, phenytoin, and rifampin alter metabolism. Always inform the lab of all supplements and medications, particularly any steroid-containing creams or supplements that may cross-react with the immunoassay.

Common Misinterpretations

A common error is reading an afternoon cortisol against morning reference ranges. A value of 6 µg/dL drawn at 4 pm is physiologically normal, but can be misinterpreted as low if the AM range is used. Conversely, an isolated mildly elevated morning cortisol without loss of circadian rhythm is often misjudged as Cushing syndrome when it actually reflects acute venipuncture stress, depression, severe obesity, alcohol withdrawal, or uncontrolled diabetes, conditions known as pseudo‑Cushing. Jumping to adrenal imaging without confirmatory testing leads to unnecessary scans and incidental findings.

Another frequent mistake is misdiagnosing adrenal insufficiency based on a single low morning cortisol in a patient who is taking glucocorticoids. Exogenous steroid use suppresses the entire HPA axis and is the most common cause of low cortisol worldwide; measuring both cortisol and ACTH distinguishes secondary adrenal insufficiency (low ACTH) from primary Addison disease (high ACTH). Additionally, total cortisol rises in pregnancy and with estrogen therapy due to increased binding globulin, yielding a “high” result without true hypercortisolism; salivary or urine free cortisol assays avoid this trap. Always pair cortisol with ACTH and, when necessary, a stimulation test to avoid misclassification.

Frequently Asked Questions

What does a high cortisol (AM) blood test mean?

A high morning cortisol level raises suspicion for Cushing syndrome, characterized by excessive adrenal hormone production. This can be ACTH-dependent, usually a pituitary adenoma, or ACTH-independent, most often caused by a benign adrenal tumor or chronic glucocorticoid use. However, pseudo‑Cushing states such as severe depression, alcoholism, and poorly controlled diabetes also elevate cortisol. A single elevated AM value is not definitive; diagnosis requires at least one confirmatory test like late‑night salivary cortisol, 24‑hour urine free cortisol, or a dexamethasone suppression test.

What causes low cortisol levels in the morning?

Low morning cortisol most commonly indicates adrenal insufficiency. Primary adrenal insufficiency (Addison disease) results from autoimmune damage to the adrenal cortex, yielding low cortisol and high ACTH, often with hyperpigmentation, hyponatremia, and hyperkalemia. Secondary adrenal insufficiency, low cortisol with inappropriately normal or low ACTH, is usually caused by prolonged glucocorticoid therapy with abrupt cessation, pituitary tumors, or pituitary surgery. A single AM cortisol below 3 µg/dL is strongly suggestive, while values above 19 µg/dL generally exclude insufficiency. Intermediate levels require an ACTH stimulation test.

How should I prepare for a cortisol AM blood test?

Schedule your blood draw between 6 am and 8 am to capture the natural circadian peak. Most labs do not require fasting, though some ask you to avoid food for one to two hours beforehand. On the morning of the test, refrain from heavy exercise, caffeine, alcohol, and smoking, as these can transiently spike cortisol. Inform the lab about all medications, especially estrogen supplements, oral contraceptives, any form of glucocorticoid (pills, inhalers, creams), spironolactone, and antiseizure drugs, because they can either artificially raise total cortisol or suppress the HPA axis.

Can stress affect a cortisol blood test?

Yes, acute physical or psychological stress, including pain from the venipuncture itself, can cause a surge in cortisol that temporarily pushes values above the normal range. This is one reason a single mildly elevated morning reading may not signal true Cushing syndrome. To reduce this effect, patients should arrive rested and sit quietly for 15 minutes before the draw. Chronic stress also elevates baseline cortisol, which can mimic a mild hypercortisolism and must be distinguished from pathological conditions through repeat testing and, if needed, salivary or urine cortisol assessments.

What is the normal range for cortisol at 8 AM?

Normal morning cortisol typically falls between 6 and 23 µg/dL when drawn around 8 am, but the exact reference interval is assay‑dependent and may vary between laboratories. Some labs report a range of 5‑25 µg/dL, while others use 7‑22 µg/dL. Because of this variability, always interpret your result against the reference range printed on your lab report. Values must also be placed in clinical context: a level within the 6‑23 µg/dL band is consistent with normal adrenal function, but further testing is needed if Cushing syndrome or adrenal insufficiency remains suspected.

Related Markers: Read Together for Full Context

BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Cortisol result: