How to Test Cortisol Levels: Blood, Saliva or Urine, When to Draw, and How to Read the Result

Cortisol follows a daily rhythm: it peaks in the half hour after waking, falls steeply through the morning, drifts lower through the afternoon and reaches its lowest point around midnight. A value that is normal at 8 AM would be alarming at 11 PM, and a value that is normal at 4 PM would be worryingly low at 8 AM. Laboratories publish separate reference ranges for morning and afternoon draws for exactly this reason, and a cortisol result without a collection time is close to uninterpretable.

The rhythm also decides which test answers which question. When the worry is too little cortisol (adrenal insufficiency), the peak is the informative moment: a morning value that is clearly normal effectively rules it out. When the worry is too much cortisol (Cushing syndrome), the informative moment is the trough: the disease shows up as a midnight level that fails to fall, which no single daytime blood draw can detect. That is why the screening tests for high cortisol are late-night saliva, 24-hour urine and the dexamethasone suppression test, not a morning blood test.

Question	Test	When to collect	What it shows
Fatigue, dizziness on standing, low blood pressure, salt craving, weight loss — could cortisol be low?	Morning blood (serum) cortisol	7–9 AM, ideally within an hour of waking	A clearly normal morning value makes adrenal insufficiency unlikely; a low value is followed by an ACTH stimulation test
A baseline, or symptoms that do not fit a pattern	Morning blood cortisol, often with a PM draw	7–9 AM and 3–5 PM	Whether the level peaks in the morning and falls by afternoon as it should
Weight gain in the face and trunk, purple stretch marks, easy bruising, high blood pressure or blood sugar — could cortisol be high?	Late-night salivary cortisol (two nights), 24-hour urine free cortisol (two collections), or a 1 mg overnight dexamethasone suppression test	Saliva at 11 PM–midnight; urine over a full 24 hours; dexamethasone at 11 PM with blood at 8 AM	Loss of the midnight trough, total daily output, or failure to suppress — the three accepted screens for Cushing syndrome
Sleep disruption, stress, "burnout"	Morning and afternoon blood cortisol	7–9 AM and 3–5 PM	A flattened or inverted curve; useful context, not a diagnosis — "adrenal fatigue" is not a recognised condition
Monitoring steroid medication, or after stopping long-term steroids	Morning blood cortisol	7–9 AM, before the day's dose	Whether the adrenal glands have recovered their own production

Through TestWell you can order the morning blood cortisol and the PM cortisol without a referral; the late-night saliva, 24-hour urine and dexamethasone tests are arranged by a clinician because they need specific collection kits or a prescribed dose.

Book the draw for 7–9 AM and keep your usual sleep schedule the night before; shift workers should tell the lab and their clinician when they woke, because the reference range assumes a conventional night's sleep. No fasting is needed. Avoid vigorous exercise that morning and arrive a few minutes early so you are seated calmly before the draw — exertion and acute stress both raise cortisol for a short time.

Medications matter more than food. Oral estrogen (combined contraceptives, menopausal hormone therapy) raises cortisol-binding globulin and therefore total cortisol, often into or above the upper range, without changing the active hormone; clinicians either account for it or test after a pause. Steroid medications cut both ways: prednisone and some other glucocorticoids cross-react in the cortisol assay and read as cortisol, while any steroid taken for weeks — tablets, inhalers, joint injections, potent skin creams — suppresses your own production and lowers the result. Tell your clinician about every steroid, including inhaled and topical ones. High-dose biotin supplements can interfere with the immunoassays most labs use; stop them two days before. Severe illness, major surgery and pregnancy raise cortisol physiologically.

Morning serum cortisol reference ranges run roughly 6–23 mcg/dL (165–635 nmol/L), with the lab's own range printed on the report; afternoon values are expected to be about half the morning value. The interpretation is asymmetric. For low cortisol, a morning value under about 3 mcg/dL strongly suggests adrenal insufficiency and one above about 15–18 largely excludes it; the band between is indeterminate and is resolved with an ACTH (cosyntropin) stimulation test, which measures how much the adrenal glands can produce when told to. For high cortisol, a raised morning value on its own proves little — stress, estrogen, obesity, depression, alcohol and poorly controlled diabetes all raise it — which is why Cushing syndrome is screened with the late-night, urine or suppression tests and confirmed with at least two abnormal results before imaging is considered.

What the number cannot do is diagnose "adrenal fatigue". The concept — that chronic stress exhausts the adrenal glands and produces low-normal cortisol — has been tested repeatedly and is not supported; the Endocrine Society and other bodies do not recognise it. Fatigue with a normal cortisol usually has a different, findable cause: thyroid, iron, B12, vitamin D, blood sugar, sleep apnoea or depression, which is why the fatigue and sleep panels test cortisol alongside those rather than alone.

Same-day medical care is needed for the picture of an adrenal crisis — severe weakness, vomiting, confusion, low blood pressure or fainting, especially in someone on long-term steroids or with a known adrenal or pituitary condition. Otherwise, a morning cortisol below the lab's range, or any high-cortisol screening result, is a reason to book a visit rather than to order the test again: the next steps (ACTH stimulation, a second screening method, imaging) are sequenced by a clinician. A normal morning cortisol with persistent symptoms is a reason to look elsewhere, and the result is still useful — it closes a door.