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IN-DEPTH: ENDOCRINOLOGY IN GERIATRICS


of ACTH. At the 10-minute time point, plasma ACTH concentrations 110 pg/mL are considered normal, concentrations between 110–200 pg/mL are equivocal, and those 200 pg/mL support the pres- ence of PPID. Transient yawning, coughing, and Flehmen responses sometimes occur in response to TRH administration but are transient and subside within minutes. One limitation of this test is that there are currently no established fall seasonal ref- erence ranges, so testing cannot be performed be- tween mid-July and mid-November. Additionally, the TRH stimulation test should not be performed until at least 24 hours following the oral sugar test, as the latter blunts ACTH responses to TRH admin- istration for reasons that are currently unknown.


Overnight Dexamethasone Suppression Test


In the normal horse, 98% of ACTH is produced by the pars distalis, which is subject to negative feed- back from exogenous glucocorticoid administration. Therefore, ACTH release and subsequent cortisol production should decrease following dexametha- sone administration. In animals with PPID, the abnormal pars intermedia continues to produce ACTH because it is not subject to negative feedback from exogenous glucocorticoids, so cortisol concen- tration fails to suppress after dexamethasone is ad- ministered. To perform this test, a baseline blood cortisol sample is collected in the late afternoon on day 1. Dexamethasone is then administered intra- muscularly at a dose of 40 g/kg. A second blood cortisol sample is collected between 15–19 hours later, on day 2. Failure of cortisol concentration to sup- press to 30 nmol/L supports the presence of PPID. One limitation of this test is that it takes 2 days to perform, yet its sensitivity is comparable to measuring resting ACTH concentration, which only requires one sample. Another limitation is that fall reference ranges have not been established for this test.


Acknowledgments


Declaration of Ethics The Author has adhered to the Principles of Veter- inary Medical Ethics of the AVMA.


Conflict of Interest


The Author is a member of the Equine Endocrinol- ogy Group, which provides recommendations on the diagnosis and management of equine endocrine dis- orders; the Equine Endocrinology Group is sup- ported by Boehringer Ingelheim.


References


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AAEP PROCEEDINGS  Vol. 64  2018 177


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