When a doctor suspects a patient may have Cushing’s syndrome they will do one of several possible screening tests. If the screening test indicates the patient may have Cushing’s syndrome, other tests will be carried out.
- 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. Whilst at home and at about 11pm the patient takes a 1mg dexamethasone tablet. Blood for cortisol measurement is taken in outpatients the following morning. In a normal person the dexamethasone (a synthetic cortisol-like drug) will cause the blood cortisol to be below normal 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 doctor need to carry out further tests. If a screening test is positive further tests are carried out to determine the cause of the Cushing’s syndrome.
- 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 healthy 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.
- CRH test. An injection of CRH is given and blood samples are taken immediately before the injection and 30 min and 60 min after the injection. In a normal person ACTH shows a maximum increase at 30 min and cortisol a maximum increase at 60 min. Patients with Cushing’s syndrome, due to an adrenal tumour or ectopic ACTH production, show little or no increase in ACTH or cortisol.