What tests will be done to diagnose Cushing’s syndrome?

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What tests will be done to diagnose Cushing’s syndrome?

Your healthcare provider will likely order several tests including:

  • 24-hour urinary cortisol test: This test measures the amount of cortisol in micrograms in your urine. Your healthcare provider will have you collect your urine over a period of 24 hours. 50-100 micrograms per day is a lot and could mean Cushing’s syndrome.
  • Midnight salivary cortisol test: Cortisol levels are known to be very low in the morning, and also low late at night. This test checks cortisol levels between 11:00pm and 12:00am. If you have Cushing’s syndrome, your cortisol levels will be unusually high during that hour.
  • Low-dose dexamethasone suppression test: Dexamethasone is a cortisol-like drug. In this test, one milligram of the drug taken orally at night and then the cortisol levels are measured the following morning. This test determines whether or not the adrenal glands responded to the dexamethasone by suppressing the amount of cortisol they secrete. If you have Cushing’s syndrome, your adrenal glands will not be suppressed, which means your cortisol levels the following morning will remain high. Cortisol levels are known to be very low in the morning. If you don’t have Cushing’s syndrome, you’ll have low levels of cortisol in the morning. If the levels are high, that means you likely have Cushing’s syndrome.
  • Blood test: A blood test will measure the ACTH levels in your blood. An adrenal tumor might be there if the levels are low. If the levels are normal or high, there could be a pituitary or ectopic tumor.
  • High-dose dexamethasone suppression test: This test is like the low-dose dexamethasone suppression test, but the dosage is eight milligrams instead of one. This test is typically performed after the low dose test shows high levels of cortisol in the morning (i.e. failure to suppress), and when blood tests show high ACTH in the blood. This test is used to determine the source of Cushing’s, specifically to differentiate between a pituitary adenoma (Cushing’s disease) and a tumor elsewhere in your body (for example in your lungs).

Once your healthcare provider has confirmed that you have Cushing’s syndrome, the next step is to determine why. Often it is medication or a tumor. If you’re on glucocorticoids, that is probably the cause, and your healthcare provider will likely decrease the dosage. If you’re not on glucocorticoids, that indicates there is a likely a tumor in your adrenal glands, pituitary gland or elsewhere. Your healthcare provider may recommend the following imaging studies to reveal the location of the tumor:

  • Cat scan (CT scan) or MRI abdomen: a CT scan or MRI may be performed to look for a tumor in your adrenal glands. These scans can be done with or without IV contrast. They are very sensitive at identifying adrenal tumors.
  • Magnetic resonance imaging (MRI) pituitary: An MRI will take a picture of your pituitary gland to see if there’s a tumor. In some cases, the MRI will not provide a perfect diagnosis. 50% of those with Cushing’s syndrome will have a “normal” MRI and 10% will have tumors unrelated to the syndrome.
  • Petrosal sinus sampling: This test finds the source of ACTH secretion. ACTH and other pituitary hormones go into the blood stream from the pituitary gland. They do so by going through two veins known as the inferior petrosal sinuses. For the procedure, the healthcare provider will insert a catheter into both veins and look for ACTH in the blood before and after the insertion of a corticotrophin-releasing hormone (CRH) at two, five and 10 minute intervals. This test has a 95% to 98% accuracy.
  • CT scan chest: If a distant tumor is suspected, a CT chest will be performed to evaluate for possible lung cancer.

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