Dogs with fully developed Cushing’s syndrome usually take on a classic “Cushingoid” appearance. This collection of clinical signs includes hair loss on the body and sparing the legs, thin skin with veins showing easily, and a “pot bellied” appearance. Click here for additional information about clinical signs of Cushing’s disease in the dog. These signs, along with increased thirst, urination and appetite will lead a veterinarian to suspect the diagnosis of Cushing’s disease. If no steroids have been given that may have caused the signs, diagnostic tests are then used to thoroughly evaluate the dog’s overall health, confirm the suspicion, and to differentiate between the pituitary-dependent (PDH) and adrenal-dependent forms (ADH). Click here to learn more about the differences between PDH and ADH Cushing’s Disease. Early or mild Cushing’s disease, however, may include subtle or nonspecific symptoms that remain undetected for some time.
Blood tests that veterinarians may use to evaluate dogs suspected of having Cushing’s Disease
A battery of routine tests should be run in all cases of suspected Cushing’s disease to help confirm the diagnosis, to test for concurrent illnesses that may also require treatment, and to rule out other illnesses that mimic Cushing’s syndrome. This “minimum database” includes a complete blood count (CBC) to evaluate oxygen carrying ability and immune system status, blood chemistry panel (including thyroid testing) to evaluate organ function and other important values, and urinalysis (UA) to rule out other causes of increased urination. Click here for a more complete explanation of these blood tests.
Additional tests that veterinarians may use to diagnose Cushing’s Disease in Dogs
Abdominal ultrasound is a form of imaging that can be extremely useful in assessing the size of the adrenal glands, which helps to differentiate between PDH and ADH. It is nonpainful, noninvasive, and allows real-time examination of every organ in the abdomen. X-rays (radiographs) of the chest are another important component of a complete work-up in order to look for concurrent abnormalities, such as lung masses.
After interpretation of the minimum database by the veterinarian confirms a suspicion of Cushing’s disease, endocrine testing should be performed for definitive diagnosis. Unfortunately, since cortisol levels vary widely throughout the day – and may be within the normal range when a sample is drawn even in dogs with Cushing’s disease – a simple test of the blood cortisol level does not reliably aid diagnosis. Useful tests assess cortisol levels over time and may involve comparing blood concentrations of cortisol before and after the administration of a drug intended to suppress or stimulate cortisol release. Urine testing can also be useful because cortisol pools in urine over hours or days, so concentrations fluctuate less rapidly than those in blood.
The Low-Dose Dexamethasone Suppression (LDDS) test is very effective for determining whether a dog has Cushing’s disease, but further tests may be necessary to differentiate PDH from ADH. In dogs with significant concurrent illness or a chance of previous prednisone administration, the ACTH-Stimulation test may be more appropriate for screening purposes. In cases where Cushing’s disease is not highly suspected, but must be ruled out, a urine Cortisol:Creatnine Ratio test may be used. Click here to learn more about any of these tests. Ask your veterinarian which test is most suitable for your pet’s situation.
How veterinarians determine if a dog has PDH or ADH
Once Cushing’s disease is confirmed, the High Dose Dexamethasone Suppression (HDDS) test can be used to differentiate PDH from ADH. This involves drawing a blood sample, administering a therapeutic dose of dexamethasone, and then drawing two more blood samples at 4-hour intervals. The amount of cortisol in each sample indicates whether the problem lies in the pituitary or adrenal gland.
In dogs with PDH, ACTH-stimulation tests may be performed once in a while to monitor response to medical treatment. It is important to remember that the signs and test results of Cushing’s disease can vary widely, and make it a surprisingly difficult disease to diagnose and manage. This means that patience and good communication with your veterinarian are essential.