Peripheral Cushing’s syndrome (PCS) is seen in middle-aged horses with obesity-associated laminitis. These horses tend to accumulate fat in the crest of the neck, over the rump, and in the sheath of male horses. Researchers are trying to understand this recently named syndrome. It was first proposed that classic Cushing’s disease–a disorder of the pituitary gland that leads to excessive cortisol levels in the blood–was causing the obesity-associated laminitis. Cortisol is primarily produced by the adrenal gland, and some of its main effects include stimulation of conversion of amino acids to glucose in the liver, elevation of blood sugar levels, and promotion of glycogen storage in the liver.

As a brief description, under normal circumstances, cortisol production is regulated by corticotropin-releasing hormone (CRH) release from the hypothalamus, which then stimulates adrenocorticotropic hormone (ACTH) release from the pituitary gland. ACTH in turn normally stimulates the release of cortisol. When there is enough cortisol in the bloodstream, CRH and ACTH secretion “backs off” through a negative feedback system. A significant and sustained rise in the level of cortisol circulating in the blood can result in clinical signs of classic Cushing’s disease, such as a long, curly hair coat and laminitis. However, when tested for classic Cushing’s disease using the overnight dexamethasone suppression test (DST), these overweight, laminitic horses/ponies tested normally, i.e., had normal pituitary gland function