Cushing's syndrome

Tests

When the clinician suspects a patient may have Cushing’s syndrome they will carry out a screening test of which there are several. If the screening test indicates the patient has Cushing’s syndrome, or when the test is borderline, other tests are carried out.

Screening tests

• A 24-hour urine test — a positive test is when the urine contains more cortisol than normal. The reliability of the test depends on the patient taking an accurate 24 hour urine collection
• An overnight dexamethasone test — the patient takes a tablet containing 1mg dexamethasone at about 11pm at home. Blood is taken the next morning in outpatients for cortisol measurement. In a normal person the dexamethasone (a synthetic glucocorticoid that acts like cortisol) will cause the blood cortisol to be low in the morning
• Blood cortisol measurements at 9am and midnight — in the normal person the cortisol level is high in the morning and low at midnight. In the patient with Cushing’s syndrome the midnight cortisol will also be high. This test is usually reserved for patients who are already in hospital. ACTH may also be measured on the 9am blood sample.

There are several reasons, such as drugs, anxiety and obesity, why the test may not be entirely reliable and the clinician will carry out more tests. If a screening test is positive further tests are carried out to determine the cause of the Cushing’s syndrome.

Further tests

• ACTH — blood is taken at 9am. ACTH will be low if Cushing’s syndrome is due to too much cortisol from an adrenal tumour or if the patient is taking synthetic steroids.
• Extended dexamethasone test — This test is used to distinguish ACTH from either an ectopic tumour or from a pituitary tumour. Dexamethasone is given at different doses, four times a day, over four days. In the normal person, dexamethasone will suppress ACTH secretion and hence cortisol secretion (see cortisol). Ectopic ACTH production will not normally suppress with high doses of dexamethasone whereas ACTH from a pituitary tumour usually does.

Non-laboratory tests

• Computed tomography (CT) scan — this is used to examine the pituitary, adrenal glands and other parts of the body for tumours
• Magnetic Resonance Imaging (MRI) — this is used to examine the pituitary and adrenal glands for tumours.