Cushing's Disease

Cushing's disease (hyperadrenocorticism) is the overproduction of the hormone cortisol by the adrenal glands that are located in the stomach near the kidneys. Cushing's disease occurs commonly in dogs. Most dogs with Cushing's disease are about 6 years old or older but sometimes occurs in younger dogs. Cortisol affects the function of many organs in the body, so the signs of Cushing's disease may be varied. Some of the more common signs of Cushing's disease include hair loss, pot-bellied appearance, increased appetite, and increased drinking and urination. Hair loss caused by Cushing's disease occurs primarily on the body, sparing the head and legs. The skin is not usually itchy as it is with other skin diseases. If you pick up a fold of skin on a dog with Cushing's disease, you may notice that the skin is thinner than normal. The dog may also bruise easily.

Less common signs of Cushing's disease are weakness, panting, and an abnormal way of walking (stiff or standing or walking with the paws knuckled over). Some dogs develop a blood clot to the lungs and may have difficulty breathing.

There are two types of Cushing's disease that are treated differently. The most common form of is caused by the overproduction of a hormone by the pituitary gland in the brain that in turn controls the amount of cortisol produced by the adrenal glands. This is called pituitary-dependent Cushing's. A small percentage of dogs with have a tumor of one of the adrenal glands which is called adrenal-dependent Cushing's.

There is no single test to diagnose Cushing's disease. The history, physical exam, and results of initial blood and urine tests often provide a strong suspicion for the presence of Cushing's disease. Laboratory tests that are most commonly altered are an increase in white blood cell count, increase in the liver enzyme ALP, increased blood sugar, increased cholesterol and dilute urine.

The large amount of cortisol in the body suppresses the immune system and allows the dog to get bacterial infections. The most common location for infection is the bladder. Dogs may have a silent bladder infection and not show signs of having the infection such as straining to urinate. A urine culture may be necessary to diagnose the infection.

X-rays of the abdomen often show a large liver. Occasionally the x-ray will show calcium in the area of one of the adrenal glands that may suggest an adrenal tumor. Ultrasound of the abdomen  may show enlargement of both adrenal glands in dogs with pituitary-dependent Cushing's or enlargement of just one of the adrenal glands in dogs with an adrenal tumor. The adrenal glands are NOT always seen during an ultrasound exam in dogs with Cushing's. In some dogs with an adrenal tumor, the tumor can be seen growing into large blood vessels close to the adrenal gland or spread from the tumor may be seen in the liver.

Specific tests for Cushing's disease are performed to confirm the diagnosis and to determine the type of Cushing's disease that is present, pituitary-dependent, or adrenal-dependent. In some cases the results are clear cut and the diagnosis is made, but in other cases the test results are not clear cut and a series of tests must be performed. Some of the specific tests for Cushing's disease include urine cortisol/creatinine ratio, low dose dexamethasone suppression test, high dose dexamethasone suppression test, and an ACTH stimulation test.

The treatment of the most common type of Cushing's disease (pituitary-dependent) is lifelong oral medication. The most common drug used to treat Cushing's disease is  Lysodren or mitotane. Occasionally ketoconazole or L-Deprenyl  is used. Initially given daily or twice daily for about a week. They can have serious side effects, so dogs being treated for Cushing's disease must be closely watched. After induction medication is given less often, usually once or twice weekly for the life of the dog. Some dogs will have a recurrence of signs of Cushing's disease later in life, even though they are receiving their medication.

Treatment of adrenal dependent Cushing's disease is by surgical removal of the cancerous adrenal gland. These tumors can spread to other parts of the body in which case all the cancer cannot be removed by surgery. Drugs such as ketoconazole are often given before surgery to reduce hormone levels before surgery. 

The prognosis for pituitary-dependent Cushing's disease with treatment is usually good. Some signs will disappear quickly and others more gradually. Appetite and water consumption usually return to normal in a few weeks where as full return of the fur may take several months.

Cushing's disease occurs more commonly than the opposite condition, Addison's disease (underproduction of cortisol) in dogs.

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