Diagnosing Cushing's syndrome can often be challenging, particularly when the symptoms are mild. It could be several months before a diagnosis is confirmed.
You may be referred for a number of different tests, as symptoms of Cushing's syndrome are similar to those of other more common conditions, such as:
Your GP will want to know whether you're taking any medication, as long-term use of corticosteroids is the main cause of Cushing's syndrome.
Tell your GP about all the medication you're 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:
The level of cortisol in saliva normally drops to low levels overnight. However, people with Cushing's syndrome show high levels late in the evening and overnight, and may be asked to collect a sample of saliva at midnight. Normally, this test is done at home and delivered to hospital in the next few days.
Before taking a blood test, you may be given a medicine called dexamethasone in tablet form. If you're otherwise healthy, dexamethasone should decrease your cortisol levels. If your cortisol levels don't decrease, it could be caused by Cushing's syndrome.
None of the above tests are completely accurate or reliable, but if you have one or more abnormal results, you'll 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's important to establish what's causing it (unless you're currently taking corticosteroids). This 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. This would suggest there's a tumour in your pituitary gland or, less commonly, in your lungs. Low levels of ACTH would suggest there's a tumour in one of your adrenal glands.
A suspected tumour can usually be confirmed by a magnetic resonance imaging (MRI) scan, which can 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. If this is the case, a further test, known as petrosal sinus sampling, may be recommended.
Petrosal sinus sampling
Petrosal sinus sampling involves a blood sample being taken from the veins of both your pituitary gland and your forearm. 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 – further scans, such as a CT scan of your chest, may be carried out to check the tissues of your lungs
It can take a considerable amount of time before 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'll need monitoring tests, and sometimes you may have to stay in hospital for a few days to adjust to 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).