What is Cushing’s Disease in Dogs?

In a healthy dog, the endocrine system is a finely tuned “machine”, composed of different “parts” (the organs) that communicate with each other via chemical signals called hormones in order to keep each other in check. One of the main organs of the endocrine system, the pituitary gland, is located in the brain and, among other hormones, secretes adrenocorticotrophic hormone (also known as ACTH). As the name implies, this hormone’s action is directed towards the adrenal glands, located far from the pituitary gland, in the area of the kidneys.

As a result of the presence of ACTH in the blood, the adrenal glands secrete glucocorticoids, or cortisone-like chemicals; specifically, they secrete cortisol. Cortisol is involved in many important processes in a healthy body, like the regulation of fat metabolism, immune responses, blood sugar levels and kidney functions. The fine-tuned endocrine system makes sure that just the right amount of cortisol is in the blood; indeed, high levels of cortisol in the blood are a signal for the pituitary to stop secreting ACTH. No more ACTH, no more cortisol. If cortisol levels are low, the pituitary steps up its secretion of ACTH. Therefore, both the adrenal glands and the pituitary glands are constantly communicating with each other to ensure a “dynamic equilibrium” of cortisol in the blood.

In canine Cushing’s disease, this balance is essentially upset. Indeed, another name that very accurately describes Cushing’s disease is hyperadrenocorticism; essentially, Cushing’s disease (also known as Cushing’s Syndrome) involves a disruption in the balance between the pituitary and the adrenal glands and an over-production of cortisol. The excessive administration of corticosteroids given to treat inflammatory diseases could cause this over-production; this is known as iatrogenic Cushing’s disease and is easily treated by simply discontinuing the medication.

Cushing’s disease in dogs can also occur naturally and has one of two causes, both involving tumors.

• The most common cause of canine Cushing’s disease is the presence of a tumor in the pituitary gland, similar to what occurs in humans. The presence of this tumor causes the excessive secretion of ACTH, no matter what the blood levels of cortisol; as more and more ACTH enters the blood, the adrenal glands respond with the non-stop production of cortisol. About 80-85% of Cushing’s disease in dogs is caused by a pituitary tumor.

• The second type of tumor to cause hyperadrenocorticism is located in the adrenal glands themselves. This tumor leads to the increased production of adrenal hormones, including cortisol, regardless of the presence or absence of ACTH. Essentially, the adrenal glands stop listening to the pituitary and continue to secrete cortisol non-stop. Between 15-20% of canine Cushing’s disease is caused by a tumor in the adrenal gland.