Causes Of Cushing's Disease In Dogs

There are three main causes of Cushing's disease in dogs. Each of these causes requires a different type of treatment approach.

Iatrogenic Hyperadrenocorticism

If your dog has allergies or another chronic condition, it is possible that this condition requires the constant administration of a treatment involving glucocorticoids. This can cause iatrogenic hyperadrenocorticism, by producing an excess of corticosteroids that flood your dog's body. This is a reaction almost identical to that of an adrenal tumor, since adrenal tumors also have a similar effect on your dog's body.

Even though the pituitary gland will attempt to cut secretion of ACTH, and the adrenal glands will attempt to stop producing so much cortisol, symptoms of Cushing's disease eventually develop if the glucocorticoid treatment is not stopped. If your dog is being affected by this cause of Cushing's disease, the adrenal glands of your dog will be shriveled, small, and atrophied.

Luckily, this is the most treatable type of Cushing's disease, since the glucocorticoid treatment is gradually withdrawn, and the symptoms of Cushing's disease eventually vanish.

Pituitary Dependent Hyperadrenocorticism

This is the most common cause of Cushing's disease in dogs. Pituitary dependent hyperadrenocorticism accounts for roughly 85% of all instances of Cushing's disease in dogs. Pituitary dependent Cushing's disease involves an excess secretion of the hormone ACTH by the pituitary gland.

ACTH is what stimulates the adrenal glands to produce more cortisol and other glucocorticoids. With this cause, a microadenoma (benign, microscopic tumor) usually forms in the pituitary gland. This tumor outputs excessive amounts of ACTH, which causes the adrenal glands to produce large amounts of cortisol.

Normally, with elevated cortisol levels, the pituitary gland would then stop producing ACTH to halt the production of cortisol. However, in pituitary dependent hyperadrenocorticism, the pituitary gland is unable to respond to the elevated cortisol levels. The adrenal glands of a dog with Cushing's disease caused by a pituitary gland tumor will usually be abnormally large, since they are working hard to produce high levels of cortisol in response to the ACTH.

Adrenal-Based Hyperadrenocorticism

In about 10-15% of instances of Cushing's disease in dogs, the cause of the hyperadrenocorticism is related to the adrenal gland. An adrenal tumor develops, usually on one of the two adrenal glands. This tumor continues producing cortisol, even if the ACTH hormone levels secreted by the pituitary gland drop. An ultrasound of a dog experiencing adrenal-based hyperadrenocorticism will show that one adrenal gland is usually abnormally shaped or extremely large in comparison to the other adrenal gland.