Related Articles * Cushing's Disease (Hyperadrenocorticism) 1 Symptoms of Cushing's Syndrome 2 What Exactly is Cushing's Disease 3 Testing: Laboratory Hints Suggesting Cushing's Syndrome 4 Testing: Confirming Cushing's Syndrome 5 Classifying Cushing's Syndrome: Pituitary vs Adrenal 6 Treatment: Pituitary Cushing's Syndrome 7 Pituitary Macroadenoma: an FAQ 8 Adrenal Tumor Treatment

Diagnosis of Dogs

The Low Dose Dexamethasone Suppression Test

Dexamethasone is a cortisone-type hormone which is used therapeutically for numerous conditions. The dog’s pituitary gland will perceive the presence of dexamethasone and shut off its stimulatory message to the adrenal glands. In the normal animal, this means that a drop in blood cortisol level will be seen 8 hours after a tiny dose of dexamethasone is given.

If a pituitary tumor is present, the pituitary is not about to shut off its stimulatory message and ignores the presence of the dexamethasone. No drop in cortisol level is seen at the end of eight hours.

THIS TEST IS CURRENTLY CONSIDERED THE MOST ACCURATE IN THE CONFIRMATION OF CUSHING’S DISEASE.

To Run This Test
Ideally this test is run in the morning. A baseline cortisol level is measured, a low dose of dexamethasone is given intravenously, and blood samples are checked again in 8 hours. Sometimes a 4 hour sample is also drawn as the pattern of suppression over the entire 8 hours may help classify the type of Cushing’s disease. The pet will require at least 8 hours in the hospital.

The ACTH Stim Test
Central to the concept of Cushing’s disease is the over-production of cortisol. It follows then that the adrenal glands of the Cushing’s patient would possess large amounts of stored hormone due to their chronic stimulation.

We have been talking about the stimulatory message sent from the pituitary gland to the adrenal glands. This message consists of a hormone called “Adrenocorticotrophic hormone” or “ACTH.”  In this test, a dose of ACTH is given to the patient. If a larger than expected, rise in cortisol levels is measured in 2 hours we may diagnose Cushing’s syndrome.

To Run This Test
Ideally, the patient is fasted overnight and the test is performed in the morning between 8 A.M. and 10 A.M. A blood sample is drawn, a dose of ACTH is given, and two hours later a second blood sample is drawn.

When Would We Run this Test?
Given that the Low Dose Dexamethasone Suppression test is more accurate, when might we run this test instead? It turns out that this is the only test that can be used if the iatrogenic form of Cushing’s disease is suspected. This test is also crucial in monitoring patients with Cushing’s disease, depending on which medications are used.

The Urine Cortisol:Creatinine Ratio
This is a screening test for Cushing’s disease; a positive test here does NOT confirm Cushing’s syndrome but a negative test here DOES rule it out. In this test a single urine sample is collected and the relative amounts of cortisol and creatinine (creatinine is a protein metabolyte that is excreted in urine constantly). If there is a high ratio (a relatively high amount of cortisol being excreted), further testing is in order.

Diagnosis of Cats

In cats, the tests that are reliable in dogs simply are not helpful. The ACTH stim test produces unacceptable false negatives and false positives. The Low Dose Dexamethasone Suppression Test cannot be performed using the same dexamethasone dosing as in dogs. (Instead the doses used for the High Dose Dexamethasone Suppression Test in dogs can be used in the Low Dose Dexamethasone Suppression Test for cats). This test will pick up most feline cases of Cushing's disease but it turns out the most reliable feline test is the least reliable canine test: the urine cortisol:creatinine ratio.

To perform this test, the owner will need to bring in a urine sample obtained in the morning. This can be most easily done by covering the litter box with clear plastic food wrap (the cat will urinate on the wrap and the urine can be collected). Alternatively, the box can be cleaned and only a small amount of litter placed inside (the urine can be collected from the bottom of the box - a few specks of litter in the sample will not matter). There is also a product called "No Sorb" litter that is made of plastic beads. It can be cleaned and re-used and your veterinarian's office may send home this or a similar product to assist with urine collection.

A total of two samples from consecutive days are needed. 

After the second day, a discrimination test to determine the type of Cushing's disease is performed. The cat is given oral dexamethasone tablets to take at 8-hour intervals (8 AM, 4 PM and midnight would be a typical schedule) and a third urine sample is brought to the vet's office.

At the end of this testing, the veterinarian will know whether the cat has Cushing's disease and will have a sense of whether it is the adrenal or pituitary form. Assuming the cat has Cushing's disease, there is one last diagnostic test and that is ultrasound. The ultrasound results must match the results of the dexamethasone suppression test.  The cat will be ready for treatment once the type of Cushing's disease has been determined.

Date Published: 1/1/2001
Date Reviewed/Revised: 04/17/2006