How Do You Diagnose Cushing’S? Step By Step

Diagnosing Cushing’s syndrome is a multi-step process that starts with confirming your body is producing too much cortisol and ends with finding the exact cause. Doctors use a combination of blood, urine, and saliva tests, followed by imaging scans to locate any tumors. The process is deliberate and takes time because high cortisol can have many causes, and a single abnormal test is never enough for a diagnosis.

What Is Cushing’s Syndrome and Why Is It Hard to Diagnose?

Cushing’s syndrome happens when your body is exposed to high levels of the hormone cortisol for a long time. Cortisol is essential for many body functions, including managing stress, controlling blood sugar, and regulating metabolism. But too much of it causes a distinct set of symptoms.

Common signs include weight gain around the midsection and face, a round “moon” face, thin skin that bruises easily, purple stretch marks, and muscle weakness. Many people also experience high blood pressure, high blood sugar, and mood changes.

The difficulty is that these symptoms overlap with many other conditions. Obesity, depression, and poorly controlled diabetes can all look similar. This is why doctors do not rely on symptoms alone. They need objective laboratory evidence of excess cortisol.

Step 1: Screening Tests to Confirm Excess Cortisol

Before looking for a cause, your doctor must first prove that your cortisol levels are actually too high. This is the most important step. Three main screening tests are used, and your doctor will likely start with one or two of them.

The 24-hour urine free cortisol test measures the total amount of cortisol your body produces over an entire day. This test is useful because cortisol levels naturally fluctuate throughout the day, and a single blood draw can miss spikes. You collect all your urine in a special container for 24 hours, and the lab measures the total cortisol.

The late-night salivary cortisol test is another reliable option. Cortisol levels normally drop to near zero at night. If your cortisol is still high late at night, that is a strong sign of Cushing’s syndrome. You spit into a small tube at bedtime, usually around 11 PM, and the lab measures the cortisol in your saliva.

The low-dose dexamethasone suppression test is the third common screening tool. You take a small dose of dexamethasone, a synthetic steroid, at night. In healthy people, this drug signals the brain to stop producing cortisol, so the next morning’s cortisol level is low. In people with Cushing’s syndrome, cortisol stays high because the system is not responding to normal feedback signals.

If any of these tests are abnormal, the doctor will repeat them. A single abnormal result can be a false positive. Two abnormal tests make the diagnosis much more secure.

Step 2: Distinguishing Cushing’s Syndrome from Cushing’s Disease

Once excess cortisol is confirmed, the next question is where the problem is coming from. This distinction matters because it changes the treatment plan completely.

Cushing’s disease is a specific type of Cushing’s syndrome caused by a benign tumor on the pituitary gland in the brain. This tumor produces too much ACTH, a hormone that tells the adrenal glands to make cortisol. Cushing’s disease accounts for about 70 percent of cases where the body makes its own excess cortisol.

Cushing’s syndrome more broadly includes other causes. An adrenal tumor on the kidney can produce cortisol directly without any signals from the brain. Rarely, a tumor elsewhere in the body, such as in the lungs, can produce ACTH. This is called ectopic ACTH syndrome.

To tell these apart, doctors measure your ACTH level in the blood. If ACTH is high, the pituitary or an ectopic tumor is driving the problem. If ACTH is low, the adrenal glands themselves are the source.

Step 3: The High-Dose Dexamethasone Suppression Test

If your ACTH is elevated, the doctor needs to know whether it is coming from the pituitary gland or from somewhere else. The high-dose dexamethasone suppression test is one way to make this distinction.

This test works on a simple principle. Pituitary tumors often remain partially responsive to feedback signals. A high dose of dexamethasone may still suppress cortisol production even though a low dose did not. Tumors outside the pituitary usually do not respond at all.

If cortisol drops by more than 50 percent after the high dose, the cause is almost certainly a pituitary tumor. If cortisol does not drop, the source may be an ectopic tumor elsewhere in the body.

This test is not perfect. Some ectopic tumors respond to dexamethasone, and some pituitary tumors do not. For this reason, many endocrinologists now prefer a different test called the CRH stimulation test, which uses a hormone to stimulate ACTH production. The results are interpreted together with imaging findings.

Step 4: Imaging Tests to Locate the Tumor

Once the blood tests point to a specific source, imaging can usually find the actual tumor. The type of scan depends on where the problem is located.

An MRI of the pituitary gland is the standard test when Cushing’s disease is suspected. The scanner creates detailed images of the brain, and the radiologist looks for a small tumor on the pituitary. These tumors are often very small, sometimes only a few millimeters wide. They can be difficult to see, and a normal MRI does not completely rule out a pituitary tumor.

A CT scan of the adrenal glands is used when the blood tests suggest an adrenal source. The CT scan can reveal tumors or nodules on the adrenal glands sitting on top of the kidneys.

If an ectopic source is suspected, the doctor may order a CT scan of the chest and abdomen to look for tumors in the lungs, pancreas, or other organs.

Step 5: Inferior Petrosal Sinus Sampling for Difficult Cases

Sometimes the tests are not clear. The MRI shows no tumor, or the blood test results are borderline. In these cases, doctors may use a procedure called inferior petrosal sinus sampling (IPSS).

This is the most accurate test for distinguishing a pituitary source from an ectopic source. A specialist inserts thin catheters into veins in the groin and guides them up to the veins that drain the pituitary gland. Blood samples are taken from these veins and from a vein in the arm at the same time.

If the ACTH level in the pituitary veins is much higher than in the arm vein, the pituitary is the source. If the levels are similar, the ACTH is coming from somewhere else.

This is an invasive procedure with some risks, including headache, bleeding, and rarely neurological complications. It is only performed when other tests have not given a clear answer. It should be done at a center with experience in the procedure.

Step 6: Confirming the Diagnosis and Planning Treatment

Once the source is identified, the diagnosis is complete. The treatment plan follows directly from the cause.

Pituitary tumors are usually removed with surgery through the nose. This is called transsphenoidal surgery. Adrenal tumors are removed with laparoscopic surgery. Ectopic tumors are treated by removing the tumor wherever it is located.

If surgery is not possible or does not work, other options include radiation therapy, medications that block cortisol production, or removing the adrenal glands entirely. The choice depends on the individual case and should be made with an endocrinologist and a surgeon who specialize in this condition.