Cortisol is tested through blood, saliva, or urine samples, and sometimes through specialized stimulation or suppression tests. The right test depends on what your doctor suspects: too much cortisol, too little, or something else affecting your adrenal function. Most people will start with one of the three basic tests, and because cortisol levels swing dramatically throughout the day, timing and preparation matter more than with most lab work.
Blood, Saliva, and Urine: The Three Basic Tests
A cortisol blood test is the most common starting point. A technician draws blood from a vein in your arm, and the whole process takes less than five minutes. Morning blood draws are standard because cortisol peaks in the early hours after waking. For adults, normal morning cortisol falls between 7 and 25 mcg/dL, while afternoon levels drop to 2 to 14 mcg/dL. That wide swing is exactly why your doctor will specify when the draw should happen, typically between 6 and 9 a.m.
A late-night salivary cortisol test is usually done at home, since it needs to capture your cortisol at its lowest point (around midnight). You’ll collect saliva using a swab or small tube. The key rule: don’t eat, drink, brush your teeth, floss, smoke, or vape for 30 minutes before the sample. This test is particularly useful for screening Cushing syndrome, because healthy cortisol drops very low at night, and a failure to drop is one of the earliest signs of excess cortisol production.
A 24-hour urine cortisol test measures total cortisol output over a full day, smoothing out the natural peaks and valleys. You’ll collect every urine sample over 24 hours into a provided container, starting and ending at roughly the same time. It’s less convenient, but it gives a comprehensive picture that a single blood draw can miss.
Doctors often order more than one type, or repeat the same test, because a single cortisol measurement can be thrown off by stress, illness, poor sleep, or even the anxiety of being in a lab.
Why Timing Matters So Much
Cortisol follows a tight daily rhythm. Your body produces the most cortisol during the last few hours of sleep and through the first hour after waking. Levels then decline steadily, hitting their lowest point about four hours before you fall asleep and staying low for the first two hours of sleep. This pattern means that a blood sample drawn at 8 a.m. and one drawn at 4 p.m. can look dramatically different in the same healthy person.
For morning blood cortisol, the standard window is around 8 a.m. Late-night salivary cortisol is collected around 11 p.m. to midnight. If your sleep schedule is unusual (shift work, for example), let your doctor know, because the reference ranges assume a conventional sleep-wake cycle.
Medications and Conditions That Skew Results
Several common medications can artificially raise or lower your cortisol readings. Estrogen, oral contraceptives, and pregnancy all increase the protein that carries cortisol in your blood, which inflates total cortisol numbers on a standard blood test even though the amount of cortisol actually available to your body hasn’t changed. Conditions like hyperthyroidism and diabetes can do the same.
On the other side, obesity, low thyroid function, testosterone use, and liver cirrhosis can lower that carrier protein, making cortisol appear falsely low on blood tests. If any of these apply to you, your doctor may choose a salivary or urine test instead, since those measure cortisol that’s already separated from the carrier protein and are less affected by these factors.
Any glucocorticoid medications (steroid creams, inhalers, pills, or injections) can suppress your body’s own cortisol production. Even topical steroids applied to large areas of skin can interfere. Your doctor will likely ask you to stop certain medications before testing, or will account for them when reading results.
Suppression and Stimulation Tests
When basic cortisol levels come back borderline or unclear, doctors use more targeted tests that push the system in one direction and measure how it responds.
Dexamethasone Suppression Test
This test checks whether your body can properly dial down cortisol production. You take a small dose of a synthetic steroid (dexamethasone) at 11 p.m., then have your blood drawn the next morning at 8 a.m. In a healthy system, the medication signals the brain to stop stimulating cortisol, and morning levels drop below 1.8 mcg/dL. If cortisol stays above that threshold, it suggests something is overriding the normal feedback loop, which is one of the hallmarks of Cushing syndrome. You can eat and drink normally before this test.
Higher-dose versions of this test exist for pinpointing the source of excess cortisol. An overnight high-dose test uses eight times more dexamethasone and looks for at least a 50% drop in cortisol. A multi-day high-dose version looks for a 90% or greater drop in urine cortisol. These help distinguish between a pituitary tumor and other causes.
ACTH Stimulation Test
This test checks the opposite problem: whether your adrenal glands can produce enough cortisol when asked to. A synthetic version of the hormone that normally tells your adrenals to make cortisol is injected, and blood is drawn at baseline, 30 minutes, and 60 minutes afterward. A healthy response is a cortisol level of at least 18 mcg/dL by the 60-minute mark. Falling short of that suggests adrenal insufficiency, the condition where your adrenals can’t keep up with your body’s cortisol needs.
This is considered the best test for confirming or ruling out adrenal insufficiency when a morning cortisol level falls in the gray zone, neither clearly normal nor clearly low.
What Abnormal Results Mean
No single cortisol test is enough for a diagnosis. Guidelines for Cushing syndrome recommend at least two abnormal screening results before moving forward, using tests like 24-hour urine cortisol, late-night salivary cortisol, or the overnight dexamethasone suppression test. For suspected adrenal insufficiency, a low morning cortisol is suggestive but rarely definitive on its own. The ACTH stimulation test is typically needed to confirm it.
If excess cortisol is confirmed, the next step involves measuring ACTH (the pituitary hormone that controls cortisol) to sort out the cause. ACTH levels below 10 pg/mL point to a problem in the adrenal glands themselves. Levels above 20 pg/mL suggest the signal is coming from the pituitary or another source. Values between 10 and 20 fall into a gray zone that requires further testing.
How to Prepare for Your Test
Preparation depends on which test you’re getting, but a few principles apply broadly. Stress, intense exercise, illness, and disrupted sleep all raise cortisol, so try to keep the day before testing as normal as possible. Arrive at your blood draw calm and on time, since rushing to the appointment can spike levels. For salivary tests, follow the 30-minute rule: nothing in your mouth for half an hour before collecting.
Tell your doctor about all medications, supplements, and hormonal treatments you’re taking. Oral contraceptives, hormone replacement therapy, and steroid medications are the most common culprits for skewed results, but even over-the-counter products can matter. Your doctor may ask you to pause certain medications before testing or choose a test method that sidesteps the interference.
If your results come back borderline, expect to repeat the test or try a different method. That’s normal, not a sign that something was done wrong. Cortisol is one of the most context-sensitive hormones in the body, and getting an accurate read sometimes takes more than one attempt.