Diagnosing Cushing’s Disease in Horses

Equine Cushing’s disease is caused by an enlargement in the pars intermedia of the pituitary gland. This condition leads to oversecretion of cortisol by the adrenal gland. Affected horses may show signs such as increased thirst and urination, loss of muscle mass, an increases susceptibility to infection, and a long hair coat that doesn’t shed normally.

Because hormone levels vary by season in healthy horses, measurements could not always guarantee an accurate diagnosis of Cushing’s. Researchers have found that measurement of adrenocorticotropic hormone (ACTH) from several resting blood samples or a thyrotropin-releasing hormone (TRH) response test are helpful in making a diagnosis and allowing horses to begin treatment. A domperidone stimulation test has been used for diagnosis, but more specific results were found by testing ACTH levels after a TRH response test, especially if ACTH readings were quite elevated and other influential factors had been eliminated, according to Jill Beech, V.M.D., a researcher from the New Bolton Center at the University of Pennsylvania.

No diagnostic test for equine Cushing’s disease is completely accurate for every horse. Regardless of test results, Beech recommends that horses should be observed for other signs of disease so that treatment can begin if a veterinarian feels it is indicated for a particular horse. Studies have shown that the history of a long hair coat that fails to shed in the spring may be as reliable as specific tests for the diagnosis of equine Cushing’s disease.