Testing for Cushing’s disease is a multi-step process that typically starts with screening tests to confirm excess cortisol, then moves to blood work and imaging to pinpoint the cause. There is no single test that can diagnose Cushing’s on its own. Most people go through at least two or three different tests over weeks or months before getting a definitive answer.
Why Testing Takes Multiple Steps
Cushing’s syndrome refers to any condition where your body has too much cortisol. Cushing’s disease is a specific type caused by a small tumor on the pituitary gland (a pea-sized gland at the base of the brain) that signals your adrenal glands to overproduce cortisol. Because several different conditions can cause the same symptoms, testing follows a structured path: first confirm that cortisol is genuinely elevated, then figure out why.
The Endocrine Society recommends against jumping straight to imaging or advanced testing. Random blood cortisol levels and one-off hormone readings are unreliable for diagnosis. Instead, doctors use specific screening tests designed to catch the patterns of cortisol overproduction.
First-Line Screening Tests
There are three main screening tests, and your doctor will typically order at least two of them. Each measures cortisol differently, and no single one is perfect on its own.
24-Hour Urinary Free Cortisol
You collect all of your urine over a full 24-hour period into a container provided by the lab. This measures the total cortisol your body excreted over the entire day, smoothing out the natural spikes and dips that happen hourly. The Endocrine Society recommends collecting at least two separate 24-hour samples on different days, since cortisol production can fluctuate. Results more than double the upper limit of normal carry higher diagnostic certainty for Cushing’s, while mildly elevated results often need further investigation.
Late-Night Salivary Cortisol
Cortisol normally drops to its lowest point late at night. In Cushing’s, it stays elevated. For this test, you chew on a small cotton swab around 11 p.m. and seal it in a tube for lab analysis. You’ll do this on two separate nights. A salivary cortisol level above roughly 3.6 nmol/L at 11 p.m. is considered elevated. At a slightly lower cutoff of 2.7 nmol/L, the test catches about 97% of true Cushing’s cases. This is one of the most convenient screening tests because you do it at home.
Overnight Dexamethasone Suppression Test
Dexamethasone is a synthetic steroid that, in a healthy person, tells the brain to temporarily shut down cortisol production. You take a small pill (1 mg) at bedtime, then have blood drawn the next morning. If your morning cortisol stays above about 2 mcg/dL, it suggests your body’s cortisol regulation isn’t responding normally, which is a red flag for Cushing’s. A longer version of this test involves taking smaller doses every six hours for two days, which some doctors prefer for borderline cases.
What Can Cause a False Positive
Several common medications speed up how quickly your body breaks down dexamethasone, which means the drug clears your system before it can do its job. The result looks like failed suppression even if you don’t have Cushing’s. Anti-seizure medications like carbamazepine, phenobarbital, and primidone are well-known culprits, as is the antibiotic rifampicin. Oral contraceptives can also interfere with cortisol measurements by raising the protein that carries cortisol in blood, leading to falsely high readings. Let your doctor know about all medications and supplements before testing.
Conditions like depression, heavy alcohol use, poorly controlled diabetes, and severe obesity can also raise cortisol levels enough to mimic Cushing’s. This state, sometimes called pseudo-Cushing’s, is one of the trickiest diagnostic challenges. A combined test that pairs dexamethasone suppression with an injection of corticotropin-releasing hormone (a substance that stimulates cortisol production) can help sort this out. In studies, this combined test reaches about 90% sensitivity and specificity when a cortisol threshold of 2.5 mcg/dL is used.
Finding the Source: ACTH Testing
Once excess cortisol is confirmed, the next question is what’s driving it. A blood test measuring ACTH, the hormone that tells the adrenal glands to make cortisol, splits the possibilities into two categories.
If ACTH is above 10 pg/mL, the problem is “ACTH-dependent,” meaning something is producing too much ACTH. This points toward either a pituitary tumor (Cushing’s disease) or, less commonly, a tumor elsewhere in the body making ACTH on its own. If ACTH is below 5 pg/mL, the problem is “ACTH-independent,” meaning the adrenal glands themselves are overproducing cortisol without being told to, usually because of an adrenal tumor. Values between 5 and 10 pg/mL fall into a gray zone that needs additional testing.
Distinguishing Pituitary From Other Sources
When ACTH levels confirm an ACTH-dependent cause, the next step is determining whether the excess ACTH is coming from the pituitary gland or from somewhere else in the body. The high-dose dexamethasone suppression test helps here. You take a larger dose of dexamethasone (8 mg total over 48 hours or as a single overnight dose), and if your cortisol drops by more than 50% from baseline, it strongly suggests a pituitary source. In studies, this criterion identified pituitary Cushing’s with sensitivity ranging from 65% to 95%, depending on the specific protocol used. An ectopic (non-pituitary) source rarely suppresses with high-dose dexamethasone.
Imaging the Pituitary Gland
An MRI of the pituitary gland is the standard imaging test once a pituitary source is suspected. MRI identifies the responsible adenoma (a small, benign tumor) in about 73% of confirmed Cushing’s disease cases. Most of these tumors are microadenomas, smaller than 10 mm, which makes them easy to miss. Up to 40% of people with confirmed Cushing’s disease have an MRI that looks normal or inconclusive.
When MRI doesn’t reveal a clear tumor, a procedure called inferior petrosal sinus sampling may be recommended. This involves threading thin catheters into the veins that drain directly from the pituitary and measuring ACTH levels there compared to a peripheral vein. A significant difference confirms the pituitary as the source, even when the tumor is too small to see on imaging. This test is done in a hospital by an interventional radiologist and is considered the gold standard for localization when imaging falls short.
What the Testing Timeline Looks Like
From first suspicion to a confirmed diagnosis, the process often takes several weeks to several months. Initial screening tests may need to be repeated if results are borderline. ACTH testing and suppression tests add another round of appointments. MRI scheduling and, if needed, petrosal sinus sampling add further time. This pace can feel frustrating, but Cushing’s is a diagnosis where getting it right matters more than getting it fast. Cortisol levels fluctuate naturally, and some pituitary tumors produce excess ACTH only intermittently, which means a single normal result doesn’t always rule the condition out.
If your symptoms are consistent with Cushing’s but initial tests come back normal, your doctor may recommend repeating the screening tests weeks or months later, particularly if you have classic features like unexplained weight gain concentrated in the face and midsection, easy bruising, wide purple stretch marks, or muscle weakness.