The effect of pergolide mesylate on adrenocorticotrophic hormone responses to exogenous thyrotropin releasing hormone in horses.
Authors: Durham A E
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary: TRH Testing and Pergolide Response in Equine PPID Understanding how diagnostic test results change during pergolide treatment remains crucial for monitoring PPID cases, yet the clinical utility of repeat TRH stimulation testing post-therapy has been poorly characterised. Durham examined 64 suspected PPID cases, comparing initial TRH responses before pergolide treatment with follow-up tests conducted within 100 days of starting therapy, with additional long-term data collected in a subset of horses. Both baseline and TRH-stimulated ACTH concentrations improved significantly within the first 100 days, with no further improvements observed beyond this timeframe—however, normalisation of previously positive tests occurred in only 24% of cases (9/38), whilst 42% (16/38) remained persistently positive despite treatment. Amongst horses showing persistent positive responses, 75% nevertheless demonstrated numerically lower ACTH concentrations post-treatment, and a third of all positive responders shifted into the equivocal range. These findings have important implications for practice: whilst pergolide effectively suppresses ACTH secretion, a normalised TRH stimulation test should not be expected in the majority of treated horses, meaning persistent or equivocal test results do not necessarily indicate treatment failure and may reflect genuine therapeutic response.
Read the full abstract on PubMed
Practical Takeaways
- •Expect TRH stimulation testing to show improvement but not necessarily normalization after pergolide therapy—most horses will show some reduction in response even if tests remain positive
- •Complete normalization of TRH test results occurs in only about 1 in 4 treated horses, so continued positive results do not necessarily indicate treatment failure if baseline ACTH values have improved
- •Allow at least 100 days on pergolide before reassessing with repeat TRH testing; further improvements beyond this period are unlikely
Key Findings
- •88% of cases showed decreased TRH-stimulated ACTH response within 100 days of pergolide treatment
- •Only 24% of horses with positive pre-treatment TRH tests normalized after treatment, while 34% improved to equivocal status
- •42% of horses with positive pre-treatment TRH tests remained positive post-treatment despite numerical improvements in 75% of these cases
- •Significant improvements in baseline and TRH-stimulated ACTH concentrations occurred within 100 days with no further improvements beyond this timeframe