Cushing's syndrome (cortisol-producing adrenal tumor)
Diagnosis and Radiological tests
There are three main questions to answer to determine if a patient has Cushing's syndromeCushing's syndrome - a disease where too much cortisol is being made:
- Is the patient producing too much cortisolCortisol - a glucocorticoid that controls glucose production and suppresses the immune system?
Collecting the urine for 24 hours and measuring the cortisol (i.e. 24 hour urine cortisol level) can tell you how much cortisol was made over the day.
- Does the patient have a low cortisol level in the late evening?
Checking a midnight salivary cortisol level measures this late evening level. This is done by asking the patient to take a saliva sample with a cotton swab just before going to bed. The swab is placed in a special tube and returned to the laboratory the next day.
- Does the patient decrease the amount of steroid produced in the body normally after giving a dose of artificial steroid?
The low dose dexamethasone suppression test will check if the body responds normally. This test is performed by giving the patient 1 mg of dexamethasone at 11 PM and measuring the blood cortisol level the next morning at 8 AM. Patients with Cushing's syndrome will not lower the amount of steroid made in the body (i.e. dexamethasone will not suppress steroid production).
Figure 6: Algorithm For Determining the Cause of Cushing's Syndrome
Once the diagnosis is made, the next step is to determine the cause of the Cushing's syndrome. The blood ACTH (adrenocorticotropic hormone) level will be tested to see if the patient's disease is ACTH-independent (i.e. caused by adrenal disease) or ACTH-dependent (i.e. caused by a pituitary adenoma or an ectopic ACTH-producing tumor). If the patient has a low ACTH (i.e. suppressed), then the adrenal glands are causing the Cushing's syndrome and the patient should have an MRI or CAT scan of the abdomen. If the ACTH level is high, the patient will have additional tests to determine if there is a pituitary adenoma or ectopic tumor.
- High dose dexamethasone suppression test
The high dose dexamethasone suppression test will check if the body lowers the cortisol production in response to large doses of artificial steroid. This test is performed by giving the patient 8 mg of dexamethasone at 11 PM and measuring the blood cortisol level the next morning at 8 AM. Patients with Cushing's disease (i.e. pituitary adenoma) may lower the amount of steroid made in the body while patients with an ectopic tumor usually will not.
- CRH stimulation test
The CRH stimulation test will see if the body can increase the amount of cortisol and ACTH made by the body in response to corticotropin releasing hormone (CRH). Patients with Cushing's disease (i.e. pituitary adenoma) should be able to increase the levels of cortisol and ACTH while patients with an ectopic tumor usually cannot.
- Inferior petrosal venous sinus sampling
This test involves threading a long, tiny plastic tube or catheter up through the veins of the body to measure the blood levels of ACTH directly from the veins that come from the pituitary. This is done through a small needle-stick in the groin area and is generally not painful.
The results of the laboratory evaluation will determine what type of imaging is needed. If the patient has adrenal Cushing's syndrome, then an abdominal CAT scan or MRI will be ordered. If a pituitary adenoma is suspected, then a CAT scan or MRI of the brain will be ordered.1-3 If an ectopic ACTH-producing tumor is suspected, then a CAT scan or MRI of the chest, abdomen, and pelvis will be ordered to locate it.