Equine Cushing’s disease, or Equine Pituitary Pars Intermedia Dysfunction (PPID), is a hormonal disorder of the pituitary gland. The pituitary gland, located at the base of the brain, becomes enlarged and overproduces hormones. Structures in the pituitary and hypothalamus may be compressed by the growth, causing loss of function.

The vast majority of horses diagnosed with PPID are greater than 15 years of age. While all breeds of horses can develop PPID, it is most common in ponies and Morgan horses.