Checking for Cushing’s disease involves a step-by-step process that starts with simple screening tests to measure your cortisol levels and, if those come back abnormal, moves into more specialized testing to pinpoint the source. The diagnostic workup typically begins after you or your doctor notice a cluster of suspicious symptoms, and it can take weeks or months to complete because results often need to be repeated for confirmation.
Symptoms That Prompt Testing
Not every sign of Cushing’s carries equal weight. Many of the most common features, like weight gain around the midsection, high blood sugar, high blood pressure, and irregular periods, overlap with conditions that are far more prevalent. The physical signs that most strongly point toward excess cortisol are wide, reddish-purple stretch marks (especially on the abdomen, thighs, or upper arms), easy bruising, and noticeable muscle weakness in the upper legs and shoulders. Fine hair growth on the forehead is another clinical clue. In children, a combination of weight gain with slowed growth is particularly telling.
Because so many symptoms overlap with other conditions, doctors generally look for a pattern of several features occurring together before ordering screening tests.
Three First-Line Screening Tests
The Endocrine Society recommends starting with one of three tests, each measuring cortisol from a different angle. Your doctor will choose based on your situation, and you may end up doing more than one.
24-Hour Urine Cortisol
You collect all of your urine over a full 24-hour period into a special container. The lab then measures the total amount of cortisol your body filtered out during that day. This gives a big-picture view of cortisol production rather than a single snapshot. Guidelines call for at least two separate collections, because cortisol output can vary day to day. A result that’s three to four times the upper limit of normal is highly suggestive, while mildly elevated values need further investigation.
Late-Night Salivary Cortisol
Cortisol normally drops to its lowest point late at night. If levels remain high around 11 p.m. to midnight, that’s a red flag. You collect a saliva sample at home by chewing on a small cotton swab, then send it to the lab. Two separate collections on different nights are recommended. In studies comparing people with Cushing’s to healthy controls, average late-night salivary cortisol was roughly 12 times higher in affected individuals, and a cutoff around 6.7 nmol/L correctly identified about 98% of cases while ruling out about 95% of healthy people.
Overnight Dexamethasone Suppression Test
This test checks whether your body’s cortisol production responds normally to a “stop” signal. You take a 1-milligram dexamethasone pill (a synthetic steroid) at 11 p.m. and have your blood drawn at 8 a.m. the next morning. In a healthy person, the pill tells the brain to dial back cortisol production overnight. A morning cortisol level below 1.8 mcg/dL is considered normal suppression. Anything above that suggests your cortisol regulation isn’t working properly and warrants further evaluation.
Why Results Sometimes Need Repeating
Cortisol is sensitive to all kinds of influences. Several categories of common medications can interfere with how your body processes dexamethasone, potentially causing a false-positive result. These include certain antidepressants, cholesterol-lowering statins, calcium channel blockers used for blood pressure, proton pump inhibitors for acid reflux, some anti-seizure medications, and atypical antipsychotics. Estrogen-containing birth control pills raise the level of cortisol-binding proteins in the blood, which can falsely elevate results on blood-based tests. Doctors often ask women to stop oral estrogen about six to eight weeks before testing.
Physical stress, depression, heavy alcohol use, obesity, and poorly controlled diabetes can also raise cortisol enough to trigger abnormal screening results without true Cushing’s being present. This is sometimes called “pseudo-Cushing’s,” and it’s one reason doctors repeat tests and use more than one method before moving forward.
Confirming the Diagnosis
If two or more screening tests come back abnormal, the next step is confirming that the excess cortisol is real and persistent. Your doctor may repeat the initial tests or combine approaches. For example, you might do both the dexamethasone suppression test and the urine collection. Consistent abnormalities across multiple tests build a stronger case. At this stage, an endocrinologist (a hormone specialist) typically takes over if one hasn’t already.
Finding the Source
Once excess cortisol is confirmed, the critical question becomes: where is it coming from? The term “Cushing’s disease” specifically refers to a tumor on the pituitary gland (a pea-sized gland at the base of the brain) that overproduces a hormone called ACTH, which in turn drives the adrenal glands to make too much cortisol. But excess cortisol can also come from adrenal tumors, tumors elsewhere in the body that produce ACTH, or long-term steroid medication use.
A blood test measuring ACTH levels helps sort this out. If ACTH is very low or undetectable, the problem is likely in the adrenal glands themselves, which are producing cortisol on their own. If ACTH is normal or elevated, the signal is coming from the pituitary or from somewhere else in the body.
Pituitary MRI
When ACTH levels point toward a pituitary source, the next step is usually an MRI of the brain focused on the pituitary gland. Here’s where things get tricky: between 36% and 64% of the small pituitary tumors responsible for Cushing’s disease are too small to show up on MRI. A visible tumor larger than 6 millimeters on MRI, combined with matching lab results, is often enough to confirm the diagnosis. But a normal-looking MRI does not rule out Cushing’s disease.
Inferior Petrosal Sinus Sampling
When the MRI is inconclusive, or when doctors need to confirm that the pituitary is truly the source, a more invasive procedure called inferior petrosal sinus sampling (IPSS) may be needed. A specialist threads thin catheters through a vein in the groin up to the small veins that drain directly from the pituitary gland. Blood samples are taken from these veins and compared to blood drawn from a vein in the arm. If the ACTH concentration near the pituitary is more than twice the level in the arm (or more than three times after a stimulation drug is given), that confirms the pituitary as the source. This test is considered the gold standard for distinguishing pituitary Cushing’s disease from other ACTH-producing sources, with accuracy above 95% in experienced centers.
How Long the Process Takes
From the first screening test to a definitive diagnosis, the process often stretches over several weeks to several months. Urine collections and saliva tests need to be repeated. Medications may need to be paused. Abnormal results require confirmation. Imaging and specialized procedures are typically scheduled in sequence, each one building on the last. Cushing’s disease in particular is considered one of the more challenging hormonal conditions to diagnose, partly because cortisol levels can fluctuate (a phenomenon called “cyclical Cushing’s”) and partly because the tumors involved are often tiny.
If your initial screening tests come back normal but your symptoms are persistent and worsening, retesting after a few months is reasonable. Some cases require multiple rounds of evaluation before the diagnosis becomes clear.