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Thirsty Dogs: Canine Cushing's Disease

10/19/2006

Cushing's disease, more properly called hyperadrenocorticism, is a common hormonal condition in dogs. It occurs in people too, although it is more rare in humans.

The disease is caused by an excessive amount of hormones produced by one or both of the adrenal glands, most commonly a hormone called cortisol. Dogs with this problem can have a variety of symptoms, including increased  thirst and urination, hunger, skin or urinary tract infections, a pot belly, thin or darkened skin, and panting. They can develop diabetes or high blood pressure, as well as other serious health problems. The symptoms and health consequences will continue to worsen over time unless the condition is treated.

What causes canine Cushing's disease?

To explain what causes canine Cushing's disease, I need to first tell you a little about how the body works. Dogs (and people) each have 2 adrenal glands, one by each kidney. The adrenal glands produce a number of hormones, including cortisol. Cortisol may sound familiar to you because we use related cortisone-type medications for many purposes in human and veterinary medicine; cortisol is the natural hormone usually produced by the body in minute amounts. The body needs cortisol to remain healthy, but too much of it, whether produced by the body or given as a medication, can have side effects.

The "boss" of the adrenal glands is another gland called the pituitary gland, located at the bottom of the brain. The pituitary gland tells the adrenal glands how much cortisol to manufacture at any given time. Therefore, Cushing's disease can be caused by a malfunction of either the adrenal glands themselves, or their boss the pituitary gland.

In most dogs with Cushing's disease, it's the pituitary gland's fault. The pituitary gland develops a little tumor (technically a small brain tumor), and this tumor tells the adrenal glands to make too much cortisol. They obediently follow instructions, and then the dog suffers from the effects of too much of the hormone. Both adrenal glands become enlarged from all the extra work they are doing. In some dogs, the pituitary tumor becomes large enough to cause neurological signs.

In a minority of dogs, one of the adrenal glands develops a tumor, and it's this adrenal tumor that produces too much cortisol. In this case, one adrenal gland will be big, and the other one will be small, since it has nothing to do with its buddy doing all the work. A skilled ultrasonographer can perfom an ultrasound exam of the adrenals, and see which situation is occurring in a particular dog. It is very important to determine which type of Cushing's disease a dog has, pituitary or adrenal, because the treatment is different.

If there is a tumor on the adrenal gland the best plan is to remove it surgically if safely possible. These adrenal tumors are malignant about half of the time. If it is the pituitary gland at fault, most dogs are treated with medication  that decreases the adrenal glands' production of cortisol (such as a drug called Lysodren or mitotane; or another called trilostane), since surgery on the pituitary gland is very difficult due to its location under the brain. This surgery is being developed for dogs however, so stay tuned. Some dogs actually require radiation therapy if their enlarged pituitary gland is causing trouble.

Therapy of Cushing's disease is very rewarding but can be quite involved, so many general veterinarians choose to refer these patients to a veterinary internal medicine specialist for treatment.

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