Cushing's syndrome can be difficult to diagnose because a variety of factors-from the time of day to a temporary illness-can lead to a higher-than-average cortisol level in people who do not have Cushing's syndrome. It also can be hard to pinpoint the cause. This is important, because treatment depends on the cause. You may need to see an endocrinologist (a doctor who specializes in hormone disorders) to diagnose or treat Cushing's syndrome.

To diagnose Cushing's syndrome, your doctor will ask about your medical history and do a physical exam. He or she can usually find out from these exams whether corticosteroid medicine is causing the condition. If it is, other tests usually are not done and your doctor will consider changing your medicine.

If you are not taking steroid medicine or your doctor believes something other than steroid medicine is causing Cushing's syndrome, you will have laboratory tests to check your level of cortisol. These tests include:

• A cortisol test, which may be done on a sample of blood or on a sample of urine collected over a 24-hour period.
• An overnight dexamethasone suppression test using a low dose of steroid (often done to reconfirm a cortisol test). This test is most commonly done on a sample of blood, but a more extensive form of the test may involve both blood and urine samples.
• A test to measure cortisol in the saliva in the evening may be done.

If the above tests show you have Cushing's syndrome, the following blood tests can help your doctor find out the amount of adrenocorticotropic hormone (ACTH) in your body and whether you have a tumor on the adrenal glands, the pituitary gland, or an organ:

• ACTH test
• Corticotropin-releasing hormone (CRH) stimulation test

If the first tests show that too much ACTH is causing Cushing's syndrome, other tests may be needed to find out its source. These include:

• Inferior petrosal sinus sampling (IPSS) to find out whether too much ACTH is being released from the pituitary gland. During IPSS, a small tube (catheter) is used to collect samples from blood vessels near the brain. If these blood samples show high levels of ACTH, it usually means that the pituitary gland is the source of excess ACTH. The IPSS is often used with the CRH stimulation test.
• Computed tomography (CT scan) and magnetic resonance imaging (MRI) of the chest or pituitary gland, which may help locate a tumor producing ACTH. But sometimes the tumor may be too small to detect with a CT scan or an MRI. A CT and MRI of the adrenal gland may also be used to locate adrenal tumors.

Blood test results showing changes in body chemistry also may point to Cushing's syndrome. These include:

• An increase in the number of white blood cells (leukocytosis).
• A low potassium level (hypokalemia).
• High blood sugar (hyperglycemia).
• Increased levels of cholesterol (hypercholesterolemia) and high blood fats (hypertriglyceridemia).
• A decrease in the time it takes for blood to clot.
• A high testosterone level.