Effect of thyrotropin-releasing hormone stimulation testing on the oral sugar test in horses when performed as a combined protocol.

Elizabeth Hodge,Alycia Kowalski,Sue Lindborg,Catherine Torcivia,Darko Stefanovski,Kelsey Hart,Nicholas Frank,Andrew van Eps

doi:10.1111/jvim.15601

Abstract

BackgroundThe use of parallel dynamic tests to identify insulin dysregulation (ID) and pituitary pars intermedia dysfunction (PPID) in horses could have better diagnostic utility than measuring baseline hormone concentrations, if the tests do not alter diagnostic interpretation of one another.HypothesisPerforming a thyrotropin‐releasing hormone (TRH) stimulation test before an oral sugar test (OST) would not affect results of OST.AnimalsTwenty‐six healthy university‐owned horses.MethodsA prospective randomized placebo‐controlled, crossover design was used to evaluate 3 OST protocols: OST alone, TRH followed by OST (TRH + OST), and placebo followed by OST (placebo + OST). Agreement for plasma insulin concentrations and diagnostic interpretation were assessed with Bland‐Altman and logistic regression analyses, respectively.ResultsBland‐Altman analysis of TRH + OST versus OST alone showed good agreement between testing protocols, with bias ± SD for insulin concentrations at baseline 0.4 ± 4.7 μIU/mL (95% limits of agreement [LOA], −8.8 to 9.7), 60 minute −0.5 ± 22.6 μIU/mL (95% LOA, −44.7 to 43.8), and 90 minute 1.9 ± 20.6 μIU/mL (95% LOA, −38.5 to 42.4) after OST, similar to placebo + OST versus OST alone. Diagnostic interpretation (positive/negative) was not different between protocols (TRH + OST versus OST alone [P = .78], placebo + OST versus OST alone [P = .77], or TRH + OST versus placebo + OST [P = .57]).Conclusions and Clinical ImportanceConcurrent testing for PPID and ID with a TRH stimulation test before an OST is an acceptable diagnostic tool for investigation of endocrinopathies in horses and allows accurate testing to be performed efficiently in 1 visit.

Highlights

Endocrinopathic laminitis is the most common form of laminitis in horses.[1,2,3,4] greater than 50% of endocrinopathic laminitis cases might go unnoticed by owners.[5]
Performing the thyrotropin-releasing hormone (TRH) stimulation test before the oral sugar test (OST) did not alter the insulin or glucose concentrations measured during the OST, nor did this combined testing protocol significantly alter the interpretation of the OST outcome based on current diagnostic cutoff criteria
The results indicate that this combined protocol is suitable for clinical diagnostic use in a population of horses with equivocal or no clinical signs of pituitary pars intermedia dysfunction (PPID), similar to the current study

Summary

| INTRODUCTION

Endocrinopathic laminitis (associated with equine metabolic syndrome [EMS] or pituitary pars intermedia dysfunction [PPID]) is the most common form of laminitis in horses.[1,2,3,4] greater than 50% of endocrinopathic laminitis cases might go unnoticed by owners.[5] early diagnostic testing should be employed to identify horses with underlying endocrinopathies to implement treatment and management strategies that prevent the development or progression of laminitis. Resting plasma ACTH and the associated reference ranges can yield falsenegative results in the earlier stages of the disease.[6,7,8] Measurement of resting plasma ACTH concentration has low sensitivity for diagnosing PPID with substantial overlap between affected and unaffected horses especially during nonautumnal seasons.[9,10] the thyrotropin-releasing hormone (TRH) stimulation test can be used for early detection of PPID in horses with resting ACTH concentrations that fall within normal reference ranges.[11]. We hypothesized that performing a TRH stimulation test before an OST would not affect OST results

| MATERIALS AND METHODS

| RESULTS

| DISCUSSION

Findings

CONFLICT OF INTEREST DECLARATION