The most common cause of Cushing's syndrome in horses is a noncancerous tumor of the adrenal cortex, called an adrenal adenoma. This type of tumor grows slowly and is usually discovered when the horse has a routine physical examination. In some cases, however, the tumor may be missed completely until the horse is examined for another reason.
Cancerous tumors of the adrenal cortex don't develop that often, but if they do, they can also cause Cushing syndrome. These tumors grow much more quickly and are usually found when the horse has symptoms such as weight loss, weakness, depression, diarrhea, fever and other signs of illness. Sometimes, a cancerous tumor will invade nearby tissues or spread to other organs.