Effects of withholding feed on thyrotropin-releasing hormone stimulation test results and effects of combined testing on oral sugar test and thyrotropin-releasing hormone stimulation test results in horses.

The research article investigates how feeding conditions and combined testing affect the results of two diagnostic tests used in horses: the thyrotropin-releasing hormone (TRH) stimulation test for pituitary pars intermedia dysfunction and the oral sugar test (OST) for equine metabolic syndrome. It concludes that combined testing affects ACTH levels in TRH stimulation tests and is not currently recommended.

Methodology

• The study was conducted on 30 adult horses. The tests carried out included TRH stimulation tests under fed and non-fed conditions, an OST on its own, and an OST combined with TRH stimulation testing.
• In the TRH stimulation tests, plasma ACTH levels were measured before (baseline) and 10 minutes after (post stimulation) intravenous TRH administration. This test is important for diagnosing pituitary pars intermedia dysfunction in horses, a common hormonal disorder.
• During the OST, plasma glucose and insulin concentrations were measured before (baseline) and 60 and 90 minutes following oral corn syrup administration. This test is used to diagnose equine metabolic syndrome, a condition similar to type 2 diabetes in humans.
• The combined testing involved initiating the TRH stimulation test right after the OST’s 60-minute post-treatment sampling. The intention was to examine how simultaneous testing affects the results of these two diagnostic tests.

Results

• The results showed that the feeding conditions, whether the horses were fed or not, did not affect median ACTH concentrations when TRH stimulation tests were performed alone.
• The median baseline ACTH level showed no difference between the standalone TRH stimulation tests (whether under fed or non-fed conditions) and those combined with the OST.
• However, when the tests were combined, the median post-stimulation ACTH concentration was considerably lower than in standalone TRH stimulation tests.
• Similarly, the mean 60-minute plasma glucose concentration was significantly lower for solitary OSTs than when combined with TRH tests. But the discrepancy was not related to TRH administration.

Conclusions

• The study’s findings suggest that combined testing impacts ACTH concentrations during TRH stimulation tests and should not be used at this time.
• This may imply that the simultaneous performance of OSTs and TRH stimulation tests may compromise the accurate diagnosis of pituitary pars intermedia dysfunction or equine metabolic syndrome in horses.
• Further research may be necessary to refine combined testing procedures or to understand fully how simultaneous testing affects the results of these diagnostic tests.