Cortisol Test: What High and Low Cortisol Levels Mean

A cortisol test measures the amount of cortisol — the body's main stress hormone — made by your two adrenal glands. What makes cortisol unusual among lab values is that it changes by the hour: it peaks in the early morning and falls to its lowest point around midnight. That diurnal rhythm is why the time of your blood draw matters as much as the number itself, and why a cortisol blood test is almost always drawn around 8 a.m. This guide explains the normal range in µg/dL, what high cortisol (toward Cushing's syndrome) and low cortisol (toward adrenal insufficiency) actually mean, and the dynamic tests doctors use when a single reading isn't enough. Cortisol is one piece of a broader hormone panel.

Key takeaways

• Cortisol is the adrenal stress hormone. It follows a strong daily rhythm — high in the morning, low at night — so a blood test is timed, usually near 8 a.m.¹²
• There is no single "normal" cortisol level: the reference range depends on the time of day and the assay. A morning draw is typically about 6–23 µg/dL, far lower in the evening.³⁴
• One reading rarely settles anything. Doctors confirm a suspected excess or deficiency with dynamic tests — the dexamethasone suppression test and the ACTH (cosyntropin) stimulation test.⁵⁶
• High cortisol that is truly abnormal points to Cushing's syndrome, which is uncommon; by far the most frequent cause is taking steroid (glucocorticoid) medication.⁷⁸
• Low cortisol points to adrenal insufficiency (Addison's disease when the adrenal gland itself fails). Its acute form, an adrenal crisis, is a medical emergency.⁹¹⁰
• Biotin supplements and steroid drugs can distort the result — tell your provider what you take before testing.²³

What is cortisol?

Cortisol is a steroid hormone produced by the outer layer (cortex) of the adrenal glands, which sit on top of each kidney. Its release is controlled by the brain: the hypothalamus signals the pituitary gland to make ACTH (adrenocorticotropic hormone), and ACTH tells the adrenals to release cortisol. When cortisol rises, it feeds back to switch that signal off — a self-regulating loop known as the HPA axis.¹

Cortisol does far more than its "stress hormone" nickname suggests. It raises blood sugar for quick energy, helps regulate blood pressure, tempers inflammation and immune activity, and influences metabolism, mood, and the sleep-wake cycle.² Crucially, its secretion follows a circadian rhythm: levels surge in the hour after you wake, peak in the early morning, and drift down through the day to a nighttime low.¹¹

That rhythm has a practical consequence. A cortisol value that looks "high" at 8 a.m. may be perfectly normal, while the same number measured at 11 p.m. would be clearly abnormal. This is why the result is always read together with the time it was drawn — and why doctors often rely on dynamic tests rather than a single snapshot.

Why the test is done

A cortisol test is not a routine screening lab. Your provider orders it when the clinical picture suggests the adrenal glands are making too much or too little hormone:¹³

• to investigate Cushing's syndrome (excess cortisol): weight gain in the face and trunk, a rounded "moon" face, purple stretch marks, easy bruising, high blood pressure, high blood sugar, muscle weakness;
• to investigate adrenal insufficiency (too little cortisol): severe fatigue, low blood pressure, unexplained weight loss, nausea, salt craving, darkening of the skin;
• to help diagnose problems of the pituitary gland, which drives adrenal output through ACTH;
• to monitor treatment of a known adrenal or pituitary disorder, or long-term steroid therapy.

Because cortisol is so easily nudged by stress, illness, and the time of day, an isolated result rarely stands on its own. It is best understood alongside your symptoms and the rest of your hormone panel.

How the test is done

A cortisol blood test is a standard venous draw from the arm. The single most important detail is timing: because levels are highest in the morning, blood is usually collected between 8 and 9 a.m., sometimes with a second, paired afternoon sample (around 4 p.m.) to document the expected daily fall.¹⁴ You may be asked to rest quietly beforehand, since physical or emotional stress can transiently raise the number.

Blood is not the only way to measure cortisol, and the alternatives matter for diagnosis:

• Late-night salivary cortisol — a saliva sample collected at bedtime, when cortisol should be at its lowest. A high late-night value is a sensitive early sign of Cushing's syndrome.⁵
• 24-hour urine free cortisol — collects all urine over a full day to estimate total daily output, smoothing out the hourly swings.⁵
• Dynamic (provocation) tests — described below, these deliberately push the HPA axis to see how it responds, and are usually what confirms a diagnosis.

Normal ranges

Cortisol reference ranges depend heavily on the time of day and the laboratory assay. As an indicative guide, a morning (8 a.m.) blood cortisol typically falls around 6–23 µg/dL, dropping to roughly half that by late afternoon and lower still at night.

Good to know: these figures are assay- and lab-dependent, and the µg/dL numbers vary between reference labs. Always trust the range printed on your report. A diagnosis of cortisol excess or deficiency is never made from one number — it rests on dynamic testing (dexamethasone suppression for suspected excess; ACTH stimulation for suspected deficiency), interpreted by a physician.⁵⁶

High cortisol (Cushing's syndrome)

A mildly high cortisol result is most often not a disease. Everyday stress, acute illness, strenuous exercise, pregnancy, and estrogen (oral contraceptives, which raise the carrier protein that binds cortisol) can all lift the measured number without any true hormone excess.³ Sustained, genuine overproduction defines Cushing's syndrome — and it is uncommon.⁷

The most common cause by far is external: taking glucocorticoid (steroid) medication such as prednisone for asthma, arthritis, or an autoimmune condition. This is called exogenous or iatrogenic Cushing's. Internal (endogenous) causes are rarer and include a pituitary tumor that overproduces ACTH (Cushing's disease), an adrenal tumor, or ACTH from a tumor elsewhere in the body.⁷⁸

Signs that raise suspicion of Cushing's syndrome include weight gain concentrated in the face ("moon face") and trunk with a fatty hump between the shoulders, wide purple stretch marks, thin skin that bruises easily, high blood pressure, high blood sugar or diabetes, muscle weakness, and, in women, irregular periods.⁸¹²

Because a single blood cortisol cannot confirm it, screening uses one or more of three tests, with a second test to confirm a positive:⁵¹³

• Overnight low-dose dexamethasone suppression test — you take 1 mg of the synthetic steroid dexamethasone at 11 p.m., and cortisol is measured at 8 a.m. Normally dexamethasone suppresses cortisol; failure to suppress suggests Cushing's.
• Late-night salivary cortisol — a high bedtime value signals a lost circadian rhythm.
• 24-hour urine free cortisol — an elevated total confirms sustained overproduction.

Low cortisol (adrenal insufficiency)

A low cortisol level suggests adrenal insufficiency — the adrenal glands are not making enough. When the adrenal gland itself is damaged, it is called primary adrenal insufficiency, or Addison's disease; when the problem lies upstream in the pituitary (too little ACTH), it is secondary.⁹ The most common trigger overall is abruptly stopping long-term steroid medication, which leaves the suppressed adrenal glands temporarily unable to restart. Other causes include autoimmune destruction of the adrenal cortex (the leading cause of Addison's) and pituitary disease.⁶¹⁰

Symptoms come on gradually and are easy to miss: persistent fatigue, low blood pressure with dizziness on standing, weight loss, nausea, muscle aches, and a salt craving. In Addison's disease specifically, the skin may take on a bronzed or tanned appearance. Diagnosis rests on a low morning cortisol confirmed by the ACTH (cosyntropin) stimulation test, in which synthetic ACTH is given and cortisol is remeasured to see whether the adrenals can respond.⁶⁹

⚠️ Adrenal crisis is an emergency. The acute form — severe weakness, vomiting, abdominal pain, confusion, and dangerously low blood pressure, often triggered by infection, injury, or surgery — can be fatal without immediate injectable hydrocortisone. People on long-term steroids or with known adrenal insufficiency should be taught "sick-day" dose rules and carry emergency instructions.⁹

Factors that affect the result

Many everyday things move cortisol, which is exactly why the test is interpreted with such care:³²

• Time of day — the single biggest factor; morning and evening values differ severalfold.
• Stress, illness, and exercise — physical or emotional stress and any acute illness push cortisol up.
• Estrogen — oral contraceptives and pregnancy raise cortisol-binding globulin, inflating the measured total.
• Steroid medications — prednisone, hydrocortisone, and inhaled or topical steroids suppress your own production and can cross-react with some assays.
• Biotin supplements — high-dose biotin (common in "hair, skin and nails" products) can falsely alter immunoassay results; stop it a few days before testing.
• Shift work and disrupted sleep — flatten or shift the normal rhythm.

Always tell your provider about your medications, supplements, pregnancy, and how you were feeling at the draw. These details change the interpretation.

When to see a doctor

See a clinician if you have a cluster of suggestive symptoms rather than one in isolation — for example, rapid central weight gain with purple stretch marks, easy bruising, and new high blood pressure (toward cortisol excess), or ongoing fatigue with low blood pressure, weight loss, and salt craving (toward deficiency). A cortisol test is ordered and read by a physician, often an endocrinologist, because a single value out of range is common and usually benign.

Seek emergency care for sudden severe weakness, vomiting, confusion, or collapse in anyone with known adrenal insufficiency or on long-term steroids — this may be an adrenal crisis.⁹ Do not stop steroid medication on your own; it must be tapered under medical supervision.

Recent research

Recent literature has sharpened both the diagnosis and the framing of cortisol disorders:

• Refining the detection of cortisol excess. Reviews and guidelines continue to endorse the dexamethasone suppression test, late-night salivary cortisol, and 24-hour urine free cortisol as first-line screens, while flagging their pitfalls — oral estrogen and variable dexamethasone absorption among them. Measuring free cortisol alongside the standard test improves how abnormal suppression results are interpreted.¹³¹⁴
• Adrenal insufficiency, better mapped. Recent reviews emphasize how often the cause is iatrogenic (steroid-related) and stress patient education to prevent adrenal crisis as a core part of care, since the acute event remains a leading source of avoidable harm.⁹⁶

This research concerns diagnosis and management; it does not support self-treatment and is not a substitute for your doctor's advice.

Frequently asked questions

Bottom line

The cortisol test measures your adrenal stress hormone, and its defining feature is a strong daily rhythm — high in the morning, low at night — which is why timing and dynamic tests matter more than any single number. High cortisol is usually harmless (stress, illness, estrogen); a genuine excess (Cushing's syndrome) is uncommon and most often caused by steroid medication. Low cortisol points to adrenal insufficiency, whose acute crisis is an emergency. No cortisol value should be read in isolation — it is the full clinical picture and the rest of your hormone panel that give the number meaning, alongside your doctor's judgment.

Sources

Official US health sources and peer-reviewed publications (PubMed) used for this guide: