b'AAEP FORUM HEIDI BANSE, DVM, PHD, DIPL. ACVIMTheHorse.com/AAEP-ForumTesting for PPID: Challenges of Interpretationituitary pars intermedia dysfunc- ACTH) during the fall is more marked P tion (PPID) is a common endocrinethan that in horses without PPID. But be-disease of older horses, affectingcause horses without PPID can have more approximately 20-30% of this popula- variable ACTH responses to TRH in the tion, though veterinarians also see it infall than in other seasons, its challenging younger horses. The disease affects partto establish fall reference ranges for this of the pituitary gland called the parstest. I recommend using resting ACTH intermedia, causing the gland to growconcentrations in the fall and response to and secrete increased levels of hormonesTRH stimulation in all other seasons to such as adrenocorticotropin (ACTH).diagnose PPID.Common clinical signs include loss ofIn addition to using seasonally ap-topline or muscle mass, delayed or absentpropriate reference ranges to interpret ALEXANDRA BECKSTETT/THE HORSEshedding, and a long hair coat, known asresults, it is important to control for other hypertrichosis. This long hair coat mightfactors, when possible. Withhold grain occur in patches or over the entire body.prior to and during testing. Ideally, testing Affected horses might also develop theshould be done on the farm to eliminate hoof disease laminitis.the stress of hospitalization. Your horse Veterinarians often base their diagnosisshould be free of other disease when test-of PPID on clinical signs, particularlying is performed. Veterinarians must keep in cases of advanced disease. However,Veterinarians can choose from several bloodblood samples chilled and ensure theyre tests that allow them to measure a horses in horses with early or equivocal clini- ACTH levels when trying to diagnose PPID. centrifuged within three hours. When-cal signs, tests can help determine PPIDever possible, the reference ranges used status.should account for geographic location Factors Influencing Test Results (e.g., the Northern vs. Southern hemi-Diagnostic Testing Many factors besides PPID status canspheres). Being consistent with labora-Tests for PPID involve measuring rest- influence adrenocorticotropin levels,tory method is also important, because ing levels of ACTH using a single sampleincluding breed, thriftiness, managementdifferent methods of measurement (or blood test or evaluating ACTH response(e.g., whether the horses diet includesdifferent laboratories, even if they are to thyrotropin-releasing hormone (TRH)grain), concurrent stress or disease,using the same methodology) can reach stimulation. The dexamethasone sup- geographic location, laboratory method,very different results. pression test is another diagnostic testsample handling and storage, and, most for PPID, but veterinarians rarely useimportantly, time of year. So, the mostConclusionit. In the TRH stimulation test, veteri- crucial factor to consider when interpret- Pituitary pars intermedia dysfunction narians collect blood for ACTH mea- ing diagnostic tests is season.remains a difficult disease to diagnose, in surement before and 10 minutes afterThe pituitary gland releases adrenocor- part because there is no gold standard administering a standard dose of TRHticotropin seasonally; hormone levels starttest. Because of the challenge of inter-intravenously. Because the TRH stimula- to rise in summer, peak in fall, and arepreting diagnostic tests, these should not tion test gauges hormone level responseat their lowest in late spring. This meansbe performed in the absence of clinical to a stimulus (i.e., its a dynamic test),horseseven those without PPIDhavesigns. Season can influence your veteri-during most of the year it is generally higher ACTH and ACTH responses tonarians diagnostic test of choice (TRH more sensitive for identifying PPID thanTRH during the fall, making test resultsstimulation or resting ACTH). A variety other tests. In horses with laminitisdifficult to interpret. Researchers haveof factors can influence reference ranges veterinarians should also evaluate insulin established reference ranges for the fall,and are an ongoing area of research. Your levels and/or the horses insulin responsetransition seasons (from summer to fallveterinarian can help you decide if and to an oral sugar test, because laminitisand fall to winter), and nonfall (winter,when to test your horse and will interpret is strongly associated with abnormalspring, summer). In horses with PPIDtest results in the context of your horses insulin regulation. the rise in ACTH (and TRH response toclinical signs. hAmerican Association of Equine Practitioners, 4033 Iron Works Parkway, Lexington, KY40511859/233-0147www.aaep.org8October 2019The Horse | TheHorse.com'