Cushing’s disease in dogs can be a bit of a challenge to recognize and diagnose. It’s clinical signs are often mistaken for normal signs of ageing, and it can take multiple rounds of testing to arrive at a definitive diagnosis. Having the best pet insurance can significantly reduce the costs associated with the diagnosis of Cushing’s disease, but diagnosis and treatment can still be time-consuming and frustrating for some dog owners.
If you or your veterinarian suspect that your dog may have Cushing’s disease, read on to learn more about this condition.
What is Cushing’s disease in dogs?
Cushing’s disease occurs when your dog’s body is exposed to excessive levels of steroids. This can be caused by the overproduction of steroids within your dog’s body, or by taking steroid medications for a long period of time.
In a healthy dog, carefully regulated amounts of cortisol (a steroid) are released from the adrenal glands. Your dog has two adrenal glands, and these small glands are located near your dog’s kidneys. The adrenal glands’ production of cortisol is carefully regulated by adrenocorticotropic hormone (ACTH), a hormone that is released from the pituitary gland in the brain. A healthy dog produces more ACTH when cortisol levels are low, to trigger more cortisol production, and produces less ACTH levels when cortisol levels are high, to slow cortisol production.
Cushing’s disease in dogs can have one of three potential causes:
- Pituitary-dependent Cushing’s disease: This is by far the most common form of Cushing’s disease, accounting for roughly 85% of canine Cushing’s disease cases. In this form of Cushing’s disease, a benign tumor in the pituitary gland produces excessive ACTH. This tells your dog’s adrenal glands to produce more cortisol, leading to elevated cortisol levels in the bloodstream.
- Adrenal-dependent Cushing’s disease: In this form of Cushing’s disease, a tumor involving one of the adrenal glands produces excessive cortisol, ignoring signals from the pituitary gland. These tumors are usually benign, but may be malignant.
- Iatrogenic Cushing’s disease: This form of Cushing’s disease is caused by the administration of excessive levels of steroids (injectable, oral, or topical medications).
All three forms of Cushing’s disease cause similar symptoms, because all result in excess levels of steroids in the body. However, these three forms of Cushing’s disease differ significantly in their treatment and long-term prognosis.
Cushing's disease for dogs: Symptoms
Symptoms of Cushing’s disease are caused by excessive levels of steroids in the bloodstream. If you have ever been on a high dose of prednisone, you can probably relate to what your dog is experiencing!
Common signs associated with Cushing’s disease in dogs include:
- Increased appetite
- Increased thirst
- Increased urination
- Panting
- Weight gain (less commonly, weight loss)
- Swollen, round belly
- Hair loss
- Muscle weakness
- Lethargy
Because Cushing’s disease is often seen in older dogs, owners may mistake the signs of Cushing’s disease for normal ageing changes. For example, owners may think their dog’s lethargy and muscle weakness are caused by arthritis, hip dysplasia, or even Lyme disease. (For more information, see A vet’s guide to hip dysplasia in dogs and A Vet’s Guide to Lyme Disease in dogs.) Cushing’s disease signs also tend to come on gradually, making them difficult to recognize.
Cushing’s disease can also predispose dogs to develop other conditions. Dogs with Cushing’s disease are more likely to develop diabetes, pulmonary thromboembolism (blood clot in the lungs), urinary tract infections, high blood pressure, and pancreatitis (see Pancreatitis in dogs: A vet’s guide). Many present to their veterinarian for one of these other conditions, and Cushing’s disease is only found after a thorough medical workup.
Cushing’s disease: Diagnosis
Multiple tests are usually required to diagnose Cushing’s disease. Your vet may begin with basic screening blood work and a urinalysis. Characteristic changes on these tests can suggest the possibility of Cushing’s, especially if your dog has clinical signs consistent with the diagnosis, but confirmation requires more specialized testing.
Cushing’s disease is often diagnosed using a low-dose dexamethasone suppression test. In this test, your dog’s blood cortisol is measured before and after a steroid injection, to assess your dog’s response to the injection. A normal dog should stop producing cortisol after receiving steroids. If your dog continues to produce cortisol after receiving steroids, this suggests Cushing’s disease.
Additional testing may be needed to differentiate between adrenal-dependent and pituitary-dependent Cushing’s disease, or look for other concurrent diseases. These tests may include additional blood tests, abdominal ultrasound (to look at the adrenal glands), urine testing, or other diagnostics.
Cushing's disease in dogs: Treatment
Pituitary-dependent Cushing’s disease is treated with medication, such as trilostane or mitotane. These medications inhibit the production of cortisol. Treatment requires careful monitoring, because your dog needs cortisol in order to survive. The goal of treatment is to find just the right dose of medication, looking for a dose that suppresses your dog’s cortisol production to a level that is not too high or too low, but “just right.”
Adrenal-dependent Cushing’s disease is treated surgically, with the removal of the affected adrenal gland. Testing will first be performed to make sure that the tumor has not metastasized, because surgery will not be beneficial if the tumor has metastasized. Fortunately, most adrenal tumors are benign and do not spread.
Iatrogenic Cushing’s disease is treated by stopping all steroid treatments. This may mean finding alternative drug protocols to manage your dog’s medical conditions.
Cushing's disease dogs: Prognosis
Dogs with pituitary-dependent Cushing’s (the most common form), typically respond well to treatment. If you can commit to administering medication and performing recommended monitoring, your dog will likely maintain a good quality of life for several years.
Adrenal-dependent Cushing’s has a variable prognosis. If your dog’s tumor has not metastasized and your dog is a good surgical candidate, your dog may do well for several years after surgery. If your dog’s adrenal tumor has metastasized, the prognosis is guarded.
Iatrogenic Cushing’s disease will resolve when you stop your dog’s steroid medications.
Summary
Cushing’s disease in dogs can be associated with a variety of symptoms. Unfortunately, these symptoms can be vague and non-specific, leading some owners to dismiss signs of Cushing’s disease as normal ageing. If you or your veterinarian do suspect Cushing’s disease, based on clinical signs or lab tests, confirmatory testing can be used to arrive at a diagnosis, and determine the cause of your dog’s Cushing’s disease. Most cases of canine Cushing’s disease are caused by a benign pituitary tumor, and can be addressed with medication.
