Pituitary pars intermedia dysfunction (PPID) is one of the most common endocrine diseases in horses, generally affecting those over the age of 15 years. It is also frequently referred to as Equine Cushing's Disease. PPID is caused by degenerative changes in an area of the brain known as the pituitary gland, hence the name of the disease. This gland is located at the base of the brain. In horses with PPID, the specific section of the pituitary gland that is most affected is called the pars intermedia. Unfortunately, in horses with PPID, changes occur within this gland, which results in increased production of a hormone called adrenocorticotropic hormone (ACTH).