Note: While Cushing's disease can contribute to other metabolic disorders (such as insulin resistance), we're going to focus solely on Cushing's disease for the purpose of this post. We'll tackle insulin resistance in another blog post!
The term Equine Cushing's Disease was coined for the similarity to the syndrome in humans and dogs. (Cushing's disease in humans was originally described by a neurosurgeon at Johns Hopkins University named Harvey Cushing.) In humans and dogs, Cushing's is most commonly caused by either 1) an adrenal tumor or 2) a tumor in the anterior lobe of the pituitary gland which causes hyperplasia (excessive growth) of the adrenal gland. Both of these cause an increase in cortisol (a steroid). In horses, Cushing's is primarily due to a tumor in the intermediate lobe of the pituitary gland, but there is no associated hyperplasia of the adrenal gland. That's why the technical name of it is Pituitary Pars Intermedia Dysfunction (PPID). Whew! If you thought that was scientific, just wait!