Evaluation of combined testing to simultaneously diagnose pituitary pars intermedia dysfunction and insulin dysregulation in horses

Abstract

Background The thyrotropin‐releasing hormone (TRH) stimulation test and the 2‐step insulin sensitivity test are commonly used methods to diagnose, respectively, pituitary pars intermedia dysfunction (PPID) and insulin dysregulation (ID). Objectives To investigate the diagnostic value of combining the TRH stimulation test and the 2‐step insulin sensitivity test to diagnose PPID and ID simultaneously. Animals Twenty‐seven adult horses, 10 control horses without PPID or ID, 5 horses with PPID only, 5 horses with ID only, and 7 horses with PPID and ID. Methods Randomized prospective study. Horses underwent a TRH stimulation test alone, a 2‐step insulin sensitivity test alone, and combined testing with simultaneous TRH and insulin injection in the same syringe. Data were compared by 2‐way repeated measures analysis of variance and 2 1‐sided tests to demonstrate equivalence. Bland‐Altman plots were generated to visualize agreement between combined and independent testing. Results The effect of combined testing on plasma adrenocorticotropic hormone, blood glucose concentration, or percentage decrease in blood glucose concentration was not significantly different from the effect obtained with independent testing. One control horse appeared falsely positive for PPID, 2 PPID‐only horses appeared falsely positive for ID, and 1 PPID and ID horse appeared falsely negative for ID when tests were performed simultaneously. Bland‐Altman plots supported the agreement between combined and independent testing. Conclusions and Clinical Importance Combining the TRH stimulation test and the 2‐step insulin sensitivity test appears to be a useful diagnostic tool for equine practitioners in the field, allowing testing of a horse for both PPID and ID simultaneously.

The effect of freeze‐thaw cycles on determination of immunoreactive plasma adrenocorticotrophic hormone concentrations in horses

“…For horses in groups 1 and 3 (without TRH stimulation), whole blood was collected into plastic blood collection tubes containing ethylenediaminetetraacetic acid (EDTA) after jugular venipuncture for measurement of endogenous plasma ACTH concentration. For horses in groups 2 and 4 (with TRH stimulation), whole blood was collected by jugular venipuncture 30 minutes after the IV injection of 1 mg of TRH (Sigma‐Aldrich Pty Ltd [subsidiary of Merck], North Ryde BC, New South Wales, Australia) 18 . Within 2 hours of blood sampling, samples were centrifuged at 1000 g at 4°C for 10 minutes and plasma was separated and placed into cryovials with no additives.…”

Clinical implications of using adrenocorticotropic hormone diagnostic cutoffs or reference intervals to diagnose pituitary pars intermedia dysfunction in mature horses

“…[9][10][11][12][13][14] To remove some of the effects of these variables and improve the detection of subclinical PPID cases, the provocative thyrotropin-releasing hormone (TRH) stimulation test has been developed and is extensively used both clinically and in research. [15][16][17] However, as with measurement of plasma baseline ACTH concentration, there is similar variability with season, age, stress, and diet with the TRH stimulation test. 18,19 Although other diagnostic tools have been explored, they have limitations and are rarely used, making determination of plasma ACTH concentration the mainstay for the diagnosis of PPID.…”

Changes Associated with the Peri-Ovulatory Period, Age and Pregnancy in ACTH, Cortisol, Glucose and Insulin Concentrations in Mares

“…It has been shown that dynamic testing is more sensitive to detect small changes in pituitary activity or insulin secretion. Performing a thyrotropin-releasing hormone stimulation test, an oral glucose test or a two-step insulin sensitivity test could have improved our assessment of the endocrine status of the mares [ 40 , 41 , 42 , 43 ]. In addition, including mares with PPID, ID or both disorders would have allowed us to identify and validate any significant changes in hormone concentrations among healthy and metabolic mares.…”