Making a successful diagnosis of Cushing’s syndrome can often be challenging, particularly when the symptoms are mild.
As Cushing’s syndrome shares symptoms with 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 take several months before a diagnosis is confirmed.
Medication history
As corticosteroid use is the leading cause of Cushing’s syndrome, your GP will ask you if you have been taking any medication.
You should give your GP a complete list of all the medication you're taking, including tablets, creams and lotions, plus any natural remedies. This is because some natural remedies contain steroids.
Testing for cortisol
You'll probably need to have one or more of the following tests, which can be used to measure the levels of cortisol in your body:
- urine test
- blood test
- saliva test – this is most accurate when it's carried out in the middle of the night, so you may be asked to stay in hospital overnight
You may also be given a medication called dexamethasone before being asked to take a urine test. If you're otherwise healthy, taking 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 a doctor who specialises in treating hormonal conditions (an endocrinologist). They should be able to confirm or rule out a diagnosis of Cushing’s syndrome.
Determining the underlying cause
Once a diagnosis of Cushing’s syndrome has been confirmed, it will be necessary to determine the underlying cause (unless you're currently taking corticosteroids because this will be the cause), to decide on your course of treatment.
The first stage is to determine whether your Cushing’s syndrome is the result of high levels of the adrenocorticotropin hormone (ACTH) in your blood. If so, this would suggest that there's a tumour in your pituitary gland or, less commonly, your lungs.
Low levels of ACTH would suggest that there's a tumour in one of your adrenal glands.
A suspected tumour can usually be confirmed by taking a computer tomography (CT) scan of the location. During a CT scan, the scanner takes a series of X-rays and a computer assembles them into a detailed image of different parts of the body.
Occasionally, despite having a CT scan, it can be difficult to determine where the tumour is located. In such circumstances, a further test, known as petrosal sinus sampling, may be recommended.
A sample of blood is taken from the veins of your pituitary gland and your forearm and the level of ACTH in both samples is 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 CT scans of your chest may be carried out to check the tissues of your lungs.
It can take a considerable amount of time before investigations are complete and a full diagnosis is made. Repeated X-rays and scans are sometimes needed.
In the meantime, 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 have to stay in hospital for a few days so that you can be monitored while taking the tablets.
You may also be given a special type of X-ray, known as a DEXA scan, which can be used to check for any weakening of your bones (osteoporosis).
