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 in dogs develops when their own body overproduces cortisol. Hyperadrenocorticism in dogs generally affects middle-aged to older animals. At any age, taking high levels of steroid dog medications or taking these medications for a long time can cause the same symptoms.
Symptoms of Cushing's Disease in Dogs
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.
Here are some common symptoms associated with Cushing’s disease in dogs:
- Increased thirst and urination (polydipsia and polyuria, respectively)
- Urinating at night or having accidents
- Increased hunger
- Increased panting
- Pot-bellied abdomen
- Fat pads on the neck and shoulders
- Loss of hair
- Lack of energy
- Muscle weakness
- Darkening of the skin
- Thin skin
- Hard, white scaly patches on the skin, elbows, etc. (associated with the disease calcinosis cutis)
What Causes Cushing's Disease in Dogs?
The most common cause of hyperadrenocorticism in dogs is a benign (non-spreading) pituitary tumor. Rarely, pituitary tumors may be malignant.
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 (high-dose or long-term) 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; to reduce inflammation; or as replacement therapy for low, naturally occurring cortisone levels. Iatrogenic Cushing’s disease is reversible by stopping or reducing the medication.
To diagnose Cushing’s disease in dogs, 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 the 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 (LDDST). 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 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. An abdominal ultrasound and chest X-rays can also be useful in determining if the tumor has spread and is malignant.