This test is used to confirm or rule out the presence of Pituitary Pars Intermedia Dysfunction (PPID, Cushing’s Disease). It is used by some vets instead of or in addition to the Endogenous ACTH test.
A blood sample is taken from the horse and the cortisol (a normal steroid hormone produced by the adrenal glands) is measured. Then, dexamethasone (a potent steroid) is injected into the horse. After 19 hours, another blood sample is taken from the horse and the cortisol level is measured again.
In a normal horse, the body responds to the shot of dexamethasone by shutting down the production of its own steroids, including cortisol. In horses with PPID this steroid reduction does not take place, and the second blood sample contains the relatively same amount of cortisol as the first.
Reasons to UseRelated Observations
This test may be somewhat more reliable year-round than the endogenous ACTH test, and the cost is comparable. The reliability of this test is well documented, and compares favorably to Endogenous ACTH.
This test requires drawing two blood samples taken 19 hours apart, which is inconvenient. Like endogenous ACTH, the reliability of the result decreases in the fall in the Northern Hemisphere because of seasonal variation in normal responses.
Personally, I am reluctant to give horses dexamethasone if they have or have had a history of laminitis. Steroids can cause or worsen laminitis. While this dose of steroids is minimal and unlikely to cause a problem in an otherwise healthy horse, there is a possibility it could worsen laminitis in a horse already predisposed.
QUESTIONS TO ASK MY VET