Pituitary Pars Intermedia Dysfunction
Pituitary pars intermedia dysfunction (PPID) or equine Cushing’s disease is the most common endocrine disorder of horses. This disorder develops in older horses when dopaminergic neurons extending from the hypothalamus to the pars intermedia (intermediate lobe) of the pituitary gland undergo oxidative degeneration. These neurons secrete dopamine, which normally inhibits the pars intermedia, so loss of dopaminergic inhibition results in hyperplasia and increased hormone production. Neoplasia develops over time, leading to the formation of functional pituitary adenomas. These tumors secrete large quantities of alpha melanocyte-stimulating hormone, the normal product of the pars intermedia, as well as adrenocorticotropic hormone (ACTH), beta-endorphin, beta-lipotropin, and corticotropin-like intermediate peptide (CLIP). Increased secretion of ACTH and CLIP results in hyperadrenocorticism.
Clinical signs of advanced PPID include hypertrichosis (commonly called hirsutism), retention of the winter haircoat, skeletal muscle atrophy, polyuria/polydipsia, laminitis, and increased susceptibility to bacterial infections. These clinical signs are relatively easy to recognize in advanced cases, but PPID is more challenging to diagnose in its earlier stages when the only signs are delayed shedding of the winter haircoat and mild muscle atrophy.