Problems in the diagnosis of pituitary adenoma (Cushing's syndrome) in horses

Thyrotropin releasing hormone (TRH) stimulation tests were carried out on a number of horses, including one clinically affected horse whose age and clinical signs suggested that a pituitary adenoma was most unlikely. The results of these tests indicated that, according to criteria published overseas, the majority of these horses had pituitary adenomas. The fact that clinically normal or affected horses may have marked increases in cortisol concentrations indicates that the TRH stimulation test is not suitable for the diagnosis of pituitary adenoma. Other tests, including alterations in glucose metabolism and, if available, ACTH measurements, are probably more useful for diagnosis. However, most diagnoses will probably rely upon the characteristic clinical signs.