According to the Animal Health Diagnostic Center at Cornell University, the most commonly used diagnostic tests for diagnosis and treatment of equine Cushing’s are the endogenous ACTH & Insulin baseline in combination, the dexamethasone-suppression test (DST), and the TRH-response tests.
Single cortisol levels are also tested but usually are normal in horses with Cushing’s syndrome and should be used in combination with one of the previous tests mentioned. Normal results for the ACTH/Insulin baseline combination, DST or TRH response tests do not preclude the presence of a pituitary adenoma but usually in horses with Cushing’s syndrome, at least one of these test results will be indicative of the abnormal condition.
The combination of ACTH (Adrenocorticotrophic Hormone) and Insulin testing along with clinical signs and other test results have been useful for the diagnosis of Cushing’s syndrome in horses. Post-treatment follow-up can also include monitoring ACTH and/or Insulin levels along with clinical signs and DST testing if necessary.
The TRH-response test is used for diagnosing Cushing’s syndrome in horses. Pituitary adenoma cells seem to lose receptor specificity for hypothalamic-releasing hormones. In most cases corticotrophs (ACTH-producing cells) are abnormally stimulated by TRH (thyrotropin-releasing hormone), with increased ACTH stimulating cortisol production by the adrenal cortex. The availability of TRH can be the limiting factor for using this test.