What Is Cushing's Disease in Dogs?
The endocrine system is the collective system of glands that produce and secrete hormones in the body, one of which is the hormone cortisol. At normal levels, cortisol performs many useful functions including helping individuals respond to stress and modulating the immune system, but too much cortisol in the body can do a lot of damage.
The condition associated with an excess of cortisol is medically referred to as hyperadrenocorticism or Cushing’s disease, and it is one of the most common endocrine disorders that affects dogs.
Cushing’s disease can develop when a dog’s own body overproduces cortisol or when a dog is given corticosteroid medications (prednisone, dexamethasone, triamcinolone, etc.) at high doses and/or over a long period of time. Hyperadrenocorticism generally affects middle-aged to older animals.
Cushing's Disease Symptoms in Dogs
Common symptoms associated with Cushing’s disease are listed below. Keep in mind that all symptoms are not apparent in every patient and that many of the signs can also be associated with other diseases. To determine whether or not a dog has Cushing’s disease, a veterinarian will need to look not just at a dog’s symptoms but also at the results of several different diagnostic tests.
- Increased thirst and urination (polydipsia and polyuria, respectively)
- Increased hunger
- Increased panting
- Pot-bellied abdomen
- Fat pads on the neck and shoulders
- Recurrent infections of skin, ears, urinary tract, etc.
- Loss of hair
- Lack of energy
- Inability to sleep (insomnia)
- Muscle weakness
- Darkening of the skin
- Appearance of blackheads on the skin
- Thin skin
- Hard white scaly patches on the skin, elbows, etc. (associated with the disease calcinosis cutis)
- Neurologic abnormalities (circling, behavioral changes, seizures, etc.)
What Causes Cushing's Disease in Dogs?
The most common cause of hyperadrenocorticism in dogs is a benign (non-spreading) pituitary tumor. Malignant tumors of the pituitary, which metastasize through the body, are a much less frequent cause of hyperadrenocorticism. When a dog’s Cushing’s disease develops because of problems within the pituitary gland, the condition is called pituitary-dependent hyperadrenocorticism (PDH). PDH is responsible for approximately 80 to 85 percent of cases of naturally occurring hyperadrenocorticism in dogs. Tumors within the adrenal gland (adrenal-dependent hyperadrenocorticism or ADH) are responsible for the other 15 to 20 percent of cases of naturally occurring Cushing’s disease in dogs. Adrenal tumors have about an equal chance of being benign or malignant.
Excessive administration of corticosteroid medications can also cause hyperadrenocorticism in dogs. These drugs are commonly used to treat allergies, immune disorders and some types of cancer, reduce inflammation, or as replacement therapy for low, naturally-occurring cortisone levels.
To diagnose Cushing’s disease, your veterinarian will first take a comprehensive health history of your dog and then perform a complete physical exam. Basic lab work like a blood chemistry profile, complete blood cell count, fecal examination, and a urinalysis will likely follow.
If based on this initial assessment your veterinarian suspects that Cushing’s disease is a likely cause of your dog’s symptoms, he or she will then run tests that are necessary to definitively diagnose the condition. The first test is often a urine cortisol:creatinine ratio. If the test results are normal, then your dog probably does not have Cushing’s disease. If your dog has a high urine cortisol:creatinine ratio, then further testing is called for since many conditions can lead to this result.
The most common test used to diagnose Cushing’s disease in dogs is the low-dose dexamethasone suppression test (LDDS). A blood sample is taken to measure a dog’s baseline cortisol level and then a small amount of dexamethasone is administered by injection. Blood cortisol levels are measured four and eight hours after the dexamethasone is given. In a normal dog, the dexamethasone injection inhibits secretion of a hormone that stimulates cortisol secretion, which leads to a decrease in circulating cortisol levels. In a dog with Cushing’s disease, cortisol is not suppressed.
Unfortunately, no one diagnostic test is definitive for Cushing’s disease in all cases. Veterinarians may need to also run an ACTH stimulation test, high-dose dexamethasone suppression test, and/or perform an abdominal ultrasound to determine whether or not a dog has Cushing’s and if so, whether PDH or ADH is to blame.