Cushing’s Disease (hyperadrenocorticism) is a hormonal disease which arises when the adrenal glands produce too much of the steroid hormone, cortisol. Whilst the hormone cortisol is needed by every cell in the body, too much cortisol can have serious harmful effects.
Cushing’s can be very hard to diagnose and is based on the symptoms, physical exam, initial general health, blood and urine tests and then more in-depth tests such as the ‘Low Dose Dexamethasone Suppression Test’ and/or the ‘ACTH Stimulation Test’.
In some cases, your vet may want to perform additional tests such as ultrasound or CT/MRI and further blood tests.
How is Cushing’s Disease Diagnosed?
Cushing’s disease can be very hard to diagnose. This is because there is not just one simple diagnostic test. The signs of Cushing’s are very vague and can be caused by a number of other diseases. Dogs suffering from other diseases/health problems may even test positive to Cushing’s, even if they don’t actually have the disease.
Initial Screening Tests
These are common tests performed first to help in the initial diagnosis of Cushing’s and also look for any other conditions which may or not be affected by excess amounts of cortisol being produced.
A full clinical history – your vet will want to know about all the symptoms your dog has been experiencing.
A full clinical examination – your vet will perform a thorough physical body examination on your dog.
A full general-health blood test is performed to check liver, kidney and pancreas function and also evaluate blood sugar levels and salt (electrolyte) levels.
A complete blood count (CBC) blood test – to check your dog’s red and white blood cells.
Urinalysis – this is a series of tests performed on a sample of your dog’s urine. They will measure the concentration of the urine (which gives an indication of how the kidneys are working), look for any glucose (sugar) which may indicate underlying diabetes, look for any bacteria (which could signify a urinary infection) and look for any other cells.
Urine Cortisol Creatinine Ratio – this is another urine test which looks at how much cortisol is being lost in the urine. The amount of cortisol is compared with another urine component, creatinine. The level of cortisol is usually higher in patients with Cushing’s (although stress and other illness can also increase this ratio.
If, after these initial screening tests have been performed, they give evidence of Cushing’s disease, your vet will want to go on to perform further, more specific tests. These are likely to include the following:
Low Dose Dexamethasone Suppression Test (LDDS)
This test will look at how your dog’s adrenal glands respond to injection with the drug dexamethasone. Dexamethasone is a synthetic cortisol (steroid).
The Low Dose Dexamethasone Suppression (LDDS) test requires 3 blood samples.
The first blood sample measures your dog’s current cortisol levels.
The second blood sample measures their cortisol level 4 hours after your dog has been injected with dexamethasone.
The third blood sample measures their cortisol level 8 hours after your dog has been injected with dexamethasone.
In dogs who don’t have Cushing’s, the results will show a decrease in the normal level of cortisol their adrenal glands produce, because the dexamethasone steroid will have stopped the adrenal gland from producing anymore.
In dogs with Cushing’s, the results should show the levels of cortisol remain higher than normal, as the dexamethasone injection fails to stop the diseased adrenal gland from producing too much cortisol. However, the dexamethasone may still prevent a diseased pituitary gland from signalling to the adrenals, so this test may not pick up all cases of pituitary dependent Cushings.
As this test takes about 8 hours in total, your dog will need to stay at the vets for the day while it is being performed.
ACTH Stimulation Test
This tests the working of the adrenal glands by injecting the hormone ACTH. The level of cortisol in the blood is tested before and after injection of the ACTH hormone and this will give an indication of whether the adrenal gland is functioning normally or producing too much cortisol.
Your vet will take a blood test first to check the current level of cortisol in your dog’s blood.
They will then inject the dog with the hormone ACTH
An hour after injection of ACTH, they will take another blood sample to measure the new level of cortisol in your dog’s blood
ACTH stimulates the adrenal glands to produce cortisol. In dogs without Cushing’s disease, you would expect the blood test taken after the injection to show a mild increase in the level of cortisol.
In dogs suffering from Cushing’s, the adrenal gland shows an exaggerated response after injection with ACTH and, in the majority of cases, will show a great increase in the level of cortisol produced.
However, dogs that have received steroids can also show this exaggerated response, and the test does not tell us whether the Cushings is caused by a pituitary or adrenal tumour, as the response is the same.
The LDDS test and the ACTH are not perfect tests and can give false results. This is why your vet may want to do both tests and will want to interpret the results in conjunction with the earlier tests and clinical picture.
Additional Investigations
In most cases, your vet will be able to reach a diagnosis of Cushing’s based on the above tests and clinical findings. However, there are some cases where the vet may want to perform additional tests to the ones described. These tests can be especially useful in helping to identify the cause of the Cushing's disease. Cushing’s can be caused by a tumour of the pituitary gland (in the brain pituitary-dependent ), a tumour of the adrenal gland (adrenal-dependent) or by your dog having been on long-term steroid medication (iatrogenic).
Imaging such as ultrasound or CT/MRI can help identify the presence and location of a tumour.
Further blood tests may also be useful in locating the tumour. These include:
Endogenous ACTH test – which measures the level of ACTH in your dog’s blood. Pituitary dependent Cushing’s will show a raised level, whereas adrenal-dependent or iatrogenic Cushing’s will show a decreased level.
High-dose Dexamethasone Suppression Test (HDDS) – injection with high dose dexamethasone will give different cortisol levels depending on the location of the tumour, helping us to tell the difference between pituitary-dependent and adrenal -dependent Cushing’s.
It is not always necessary to locate the cause of your dog’s Cushing’s Disease. Your vet will be able to discuss this with you based on the previous clinical findings.
Cushing’s disease is complicated, and all cases are different, so if you wish to discuss your dog’s Cushing’s disease or their symptoms and diagnosis, one of our vets will be happy to help!
