Diagnosing 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 pituitary gland as the source. No single test can confirm it on its own, and the full workup often takes weeks or longer because results can overlap with other conditions. Here’s how the process works from start to finish.
Why Diagnosis Takes Multiple Steps
Cushing’s disease specifically refers to a small tumor on the pituitary gland 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, long-term steroid use, or even non-tumor conditions like severe obesity, depression, and chronic alcoholism. Doctors need to first prove cortisol is genuinely elevated, then figure out exactly where the problem originates. That distinction matters because the treatment for each cause is different.
Step 1: Screening for Excess Cortisol
The first goal is to answer a simple question: is your body making too much cortisol? Three screening tests are commonly used, and most doctors will order at least two of them because no individual test is perfectly accurate.
Late-Night Salivary Cortisol
Cortisol normally drops to its lowest point late at night. In Cushing’s disease, it stays elevated. You collect a saliva sample at home before bed, usually on two or more consecutive nights. A result above roughly 10 nmol/L raises suspicion, with about 94% sensitivity at that threshold. This test is considered the easiest for patients to complete since it doesn’t require a blood draw or hospital visit.
24-Hour Urinary Free Cortisol
You collect all your urine over a full 24-hour period, and the lab measures how much cortisol your body excreted. Normal ranges are roughly 5 to 64 mcg per day for men and 6 to 42 mcg per day for women. People with Cushing’s syndrome typically exceed 100 mcg per day, though there’s wide variation. Doctors usually want the average of two to three separate collections to get a reliable picture.
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 1 mg pill at 11 p.m. and have your blood drawn at 8 a.m. the next morning. If your morning cortisol stays above 1.8 mcg/dL, your body isn’t responding to that “shut off” signal the way it should. Levels above 5 mcg/dL point to significant overproduction. This test has about 95% sensitivity at the 1.8 mcg/dL cutoff. It works less reliably for shift workers or women taking estrogen-containing birth control, so doctors may choose a different test in those situations.
Step 2: Finding the Source With ACTH Levels
Once excess cortisol is confirmed, the next question is whether the problem is “ACTH-dependent” or “ACTH-independent.” A morning blood draw measuring ACTH levels helps sort this out. In Cushing’s disease, ACTH is elevated because the pituitary tumor keeps producing it. When the cause is an adrenal tumor instead, ACTH drops very low because the excess cortisol suppresses the pituitary on its own.
In one large study, patients with Cushing’s disease had a median ACTH of about 22 pmol/L, compared to just 3.1 pmol/L in those with adrenal tumors. An ACTH level below the detectable limit essentially rules out a pituitary cause. However, there’s an overlap zone between roughly 3.5 and 15.6 pmol/L where the result alone isn’t conclusive, and additional testing is needed.
Step 3: Distinguishing Pituitary From Ectopic Sources
If ACTH is elevated, the cortisol overproduction is being driven by something making too much ACTH. That’s usually a pituitary tumor (Cushing’s disease), but in a minority of cases, a tumor somewhere else in the body, often in the lungs or pancreas, can produce ACTH as well. Telling these apart is one of the trickiest parts of the diagnostic process, and results are discordant in up to one-third of patients.
CRH Stimulation Test
CRH is a hormone that normally triggers ACTH release from the pituitary. When injected during a test, patients with Cushing’s disease usually show a spike in both ACTH and cortisol. Ectopic tumors generally don’t respond the same way, though well-differentiated tumors can sometimes mimic the pituitary response, creating false positives.
High-Dose Dexamethasone Suppression Test
This uses the same principle as the screening version but with a much larger dose of 8 mg. Pituitary tumors often retain some sensitivity to feedback, so cortisol levels typically drop by more than about 50% in Cushing’s disease. Ectopic tumors rarely suppress. The accuracy isn’t perfect, so this test is often used alongside the CRH stimulation test rather than on its own.
MRI of the Pituitary Gland
MRI is the primary imaging tool for locating the pituitary tumor. Most ACTH-producing tumors are microadenomas, meaning they’re smaller than 10 mm. Across studies, MRI detects these tumors with a sensitivity of about 71%, which means roughly 30% of confirmed cases show no visible tumor on the scan. Machines with stronger magnets (3 Tesla vs. the standard 1.5 Tesla) improve detection when available.
A visible tumor on MRI that matches the biochemical evidence can sometimes be enough to proceed to treatment. But when the MRI is negative or shows something ambiguous, doctors need a more definitive test to confirm the pituitary as the source before surgery.
Inferior Petrosal Sinus Sampling
This is the most invasive part of the diagnostic workup, but it’s considered the gold standard for confirming that excess ACTH is coming from the pituitary. A radiologist threads thin catheters through the veins in your groin up to the petrosal sinuses, which are the veins that drain blood directly from the pituitary gland. Blood samples are taken from these veins and compared to blood drawn from a vein in your arm.
If the ACTH concentration near the pituitary is at least twice as high as in the arm (or three times as high after stimulation with CRH), the pituitary is confirmed as the source. A ratio below those thresholds suggests the ACTH is coming from somewhere else in the body. Both false positives and false negatives have been reported, and it’s essential that cortisol is actually elevated at the time of the procedure for the results to be meaningful. This test is typically performed only at specialized endocrine centers because of its technical complexity and the small risks involved with the catheterization.
The Pseudo-Cushing’s Problem
Several conditions can cause mildly elevated cortisol and even some physical features of Cushing’s without an actual tumor being present. Depression, eating disorders, severe obesity, insulin resistance, extreme physical stress, and chronic heavy drinking can all activate the body’s stress hormone axis enough to produce borderline or even moderately abnormal screening results. This is sometimes called pseudo-Cushing’s, and it’s one of the main reasons doctors require multiple positive tests before moving forward with invasive procedures. The cortisol elevation in pseudo-Cushing’s tends to be milder than in true Cushing’s disease, but the overlap in both lab values and physical symptoms like weight gain and glucose intolerance can make the distinction genuinely difficult.