How Doctors Screen for Cortisol Excess and Why One Abnormal Test Is Not a Diagnosis

Doctors screen for cortisol excess with three first-line tests: the overnight dexamethasone suppression test, late-night salivary cortisol, and 24-hour urinary free cortisol. Each is sensitive but none is perfect, which is why the Endocrine Society clinical practice guideline asks for two different tests to agree before anyone calls the result Cushing syndrome. A single abnormal value, by itself, is a reason to look further, not a diagnosis. Many benign conditions raise cortisol enough to fail one of these tests without any underlying tumor.

This article is educational and is not medical advice.

What the body is actually being asked

Cortisol follows a daily rhythm. It peaks in the early morning, falls through the day, and reaches its lowest point around midnight. It also responds to a feedback signal: give a small dose of the synthetic steroid dexamethasone and a healthy pituitary gland reads it as a cortisol signal and dials down its own output. Cushing syndrome is a state of genuine, autonomous cortisol excess, most often from a pituitary or adrenal tumor, or from cortisol-like medications taken for other conditions.

Each screening test probes a different weakness of that system. The logic matters more than the numbers, so it helps to see what each test is looking for.

The three first-line tests

Overnight dexamethasone suppression test

The patient takes 1 mg of dexamethasone at about 11 pm and has blood drawn for cortisol the next morning. In a normal axis, the pituitary suppresses and morning cortisol falls low. The Endocrine Society guideline authored by Nieman and colleagues in the Journal of Clinical Endocrinology and Metabolism sets the abnormal threshold at a serum cortisol above 1.8 micrograms per deciliter (50 nmol per liter). Failure to suppress below that line is a signal, not a verdict.

Late-night salivary cortisol

This test exploits the midnight low point. In cortisol excess, that nighttime trough disappears. Saliva collected between 11 pm and midnight, usually on two separate nights, captures the free, biologically active fraction of cortisol without a blood draw. A result above the reporting laboratory's reference range counts as abnormal. The exact cutoff varies from lab to lab because it depends on the assay method, which is one reason the guideline asks for repeat sampling rather than trust in a single figure.

24-hour urinary free cortisol

Collecting all urine over a full day integrates cortisol output across the whole period rather than at one instant. The guideline recommends at least two separate 24-hour collections, because a single one is easy to get wrong through over- or under-collection. A result above the laboratory's upper limit of normal counts as abnormal.

The 2008 guideline treats these as broadly interchangeable starting points, and the 2021 Pituitary Society consensus update on Cushing's disease led by Fleseriu and colleagues in The Lancet Diabetes and Endocrinology reaffirms the same menu. The choice among them is shaped by the individual patient, which is where pretest probability enters.

Why one abnormal test is not a diagnosis

Screening tests are tuned to catch nearly everyone who has the disease, which is the right design goal when the disease is serious and treatable. The cost of that sensitivity is that healthy people and people with unrelated conditions sometimes cross the threshold. Several well-documented situations produce a high cortisol reading with no tumor behind it.

Estrogen-containing oral contraceptives raise cortisol-binding globulin, the protein that carries cortisol in blood. Because standard serum assays measure total cortisol, that inflates the measured value and can produce a false-positive dexamethasone suppression result; the Endocrine Society guideline reports this false-positive pattern in roughly half of women taking the pill and advises stopping estrogen weeks before testing. Anticonvulsants such as phenytoin and carbamazepine speed up dexamethasone metabolism, so the dose clears before it can suppress the axis. Drinking very large fluid volumes inflates urinary cortisol. Smoking or eating licorice before a saliva sample skews that test.

Beyond medications, a group of physiologic states can genuinely activate the stress axis. Poorly controlled diabetes, heavy alcohol use, severe obesity, depression and other psychiatric illness, and acute illness all push cortisol up. This category was long called pseudo-Cushing and is now more precisely termed non-neoplastic hypercortisolism, a framing detailed by Findling and Raff in the Journal of the Endocrine Society: real cortisol elevation driven by an overactive but structurally normal system, not by a tumor. Because these patients can share clinical and biochemical features with true Cushing syndrome, distinguishing them takes time and repeat testing rather than a single lab draw.

Pretest probability and the two-test rule

Pretest probability is the estimated likelihood that a person has the disease before any lab result comes back, based on their clinical picture. The guideline is deliberate about who should even be tested: people with features that are unusual for their age, such as early osteoporosis or hypertension, and people with several progressive and more specific findings, including reddish-purple stretch marks, easy bruising, a rounded flushed face, and weakness of the muscles closest to the trunk. Testing a broad, low-risk population instead guarantees that most abnormal results will be false alarms, because when a condition is rare, even a fairly accurate test throws off more false positives than true ones.

That is the reasoning behind the two-test structure. One abnormal screen prompts referral and a second, different test. Two abnormal results that agree justify moving on to work out the cause with ACTH measurement and imaging. Two normal results effectively close the question. Discordant results call for more testing or specialist judgment rather than a coin flip. The 2021 consensus adds a practical step for suspected non-neoplastic hypercortisolism: treat the underlying condition and repeat the clinical assessment and first-line testing three to six months later, since resolving the trigger often lets the axis normalize on its own.

None of this is bureaucratic caution. It is how the testing sequence is built to fail safely, catching real disease while refusing to let a single number, produced by a birth control pill or a stressful week, become a label a patient carries for years.