Cushing's syndrome - Diagnosing Cushing's syndrome

Diagnosing Cushing’s syndrome can often be challenging, particularly when the symptoms are mild.

As the symptoms of Cushing’s syndrome are similar to those of other, more common conditions, such as an underactive thyroid gland and high blood pressure (hypertension), you may be referred for a number of different tests. As a result, it could be several months before Cushing's syndrome is confirmed.

Your GP will want to know whether you are taking any medication because the use of corticosteroids is the main cause of Cushing’s syndrome.

Tell your GP about all the medication you are taking, including tablets, creams and lotions, plus any natural remedies. Some natural remedies contain steroids.

Testing for cortisol

If Cushing's syndrome is suspected, the amount of cortisol in your body will be measured using one or more of the following tests:

• urine test
• blood test 
• saliva test (see below)

A saliva test is most accurate when it is carried out in the middle of the night, so you may be asked to stay in hospital overnight.

Before taking a urine test, you may be given a medication called dexamethasone. If you are otherwise healthy, dexamethasone should decrease your cortisol levels. If your cortisol levels are unaffected, it could be due to Cushing’s syndrome.

None of the above tests are completely accurate or reliable, but if you have one or more abnormal results, you will usually be referred to an endocrinologist (a doctor who specialises in treating hormonal conditions). They should be able to confirm or rule out a diagnosis of Cushing’s syndrome.

Finding the underlying cause

After being diagnosed with Cushing’s syndrome, it will be necessary to establish the underlying cause (unless you are currently taking corticosteroids). Finding out what is causing Cushing's syndrome will help determine your course of treatment.

The first stage is to find out whether Cushing’s syndrome is being caused by high levels of the adrenocorticotropin hormone (ACTH) in your blood.

If it is, it would suggest that there is a tumour in your pituitary gland or, less commonly, in your lungs. Low levels of ACTH would suggest that there is a tumour in one of your adrenal glands.

A suspected tumour can usually be confirmed by a magnetic resonance imaging (MRI) scan, which uses a strong magnetic field and radio waves to produce detailed images of the pituitary gland.

Occasionally, despite having an MRI scan, it can be difficult to determine exactly where the tumour is located. In such circumstances, a further test, known as petrosal sinus sampling, may be recommended.

A blood sample will be taken from the veins of your pituitary gland and your forearm, and the level of ACTH in both samples will be compared. If the level is higher in the pituitary vein, the cause is probably a tumour in your pituitary gland. If levels are similar, the cause is probably a tumour elsewhere in your lungs. Further X-rays and MRI scans of your chest may be carried out to check the tissues of your lungs.

It can often take a considerable amount of time before all of the investigations have been completed and a full diagnosis is made. Repeated X-rays and scans are sometimes required.

While waiting for the test results, your doctor may decide to treat your Cushing's syndrome with medication, such as metyrapone, to reduce the amount of cortisol. If this is the case, you will have to stay in hospital for a few days so that you can be monitored while taking the tablets.

You may also have a special type of X-ray called a DEXA scan, which can be used to check for any weakening of your bones (osteoporosis).