Cushing Syndrome (Hyperadrenocorticism)

Cushing syndrome (hyperadrenocorticism) may be the most frequent endocrinopathy in adult to aged dogs but is infrequent in other domestic animals.  The clinical signs and biochemical abnormalities result primarily from chronic excess production of cortisol. Increased cortisol levels in dogs may result from one of several mechanisms. The most common is an adenoma or hyperplasia of the adrenocorticotropic hormone (ACTH)-containing cells of the pituitary gland (pars distalis or pars intermedia), which results in bilateral adrenal cortical hypertrophy and hyperplasia. This form of the disease is referred to as pituitary-dependent hyperadrenocorticism (Cushing disease) and is seen in ~90% of cases. Functional adrenal tumors, a far less frequent cause of hyperadrenocorticism in dogs, may secrete cortisol or sex steroids, resulting in a variety of clinical signs. Many of the clinical signs and biochemical abnormalities seen with naturally occurring hyperadrenocorticism can be induced by longterm, daily administration of large doses of corticosteroids. Dogs develop a spectrum of clinical signs and laboratory abnormalities as a result of the combined gluconeogenic, lipolytic, protein catabolic, and anti-inflammatory effects of the glucocorticoid hormones on many organ systems. The disease is insidious and slowly progressive. (For discussion of the clinical signs, laboratory abnormalities, diagnosis, and treatment of hyperadrenocorticism, see The Pituitary Gland.)