Prospective Case Series of Clinical Signs and Adrenocorticotrophin (ACTH) Concentrations in Seven Horses Transitioning to Pituitary Pars Intermedia Dysfunction (PPID)
Authors: Kirkwood Naomi C., Hughes Kristopher J., Stewart Allison J.
Journal: Veterinary Sciences
Summary
# Editorial Summary: Early Detection of PPID Through TRH Stimulation Testing Diagnosing early-stage pituitary pars intermedia dysfunction (PPID) remains clinically challenging because horses often present with subtle or absent clinical signs whilst displaying borderline ACTH concentrations, delaying intervention until disease progression becomes obvious. This prospective case series tracked seven horses with equivocal basal ACTH levels over four years, recording sequential basal and TRH-stimulated ACTH measurements alongside clinical observations to document the natural transition into confirmed PPID. Two horses developed marked hypertrichosis with consistently elevated basal ACTH; four showed only intermittent mild coat changes with equivocal basal concentrations (47.4% above diagnostic cut-off); and one remained clinically normal throughout—demonstrating considerable heterogeneity in disease presentation and progression. Critically, TRH-stimulated ACTH testing proved far more discriminatory than basal sampling, exceeding diagnostic thresholds in 57.9% of measurements across the five horses with subtle signs, with particularly robust positive results during late summer and early autumn (February–March in the Southern Hemisphere). For practitioners managing early PPID suspects, TRH stimulation testing appears substantially more reliable for confirming transitional disease when basal ACTH concentrations remain equivocal, particularly if sampling occurs during seasonal peaks, enabling earlier treatment initiation and potentially preventing progression to overt clinical dysfunction.
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Practical Takeaways
- •If a horse has equivocal basal ACTH concentrations, use TRH-stimulated ACTH testing to improve detection of subclinical or transitional PPID, especially in late summer and early autumn months
- •Monitor horses with borderline ACTH results serially rather than making single-point diagnostic decisions, as some horses show progressive increases over time before developing clinical signs like hypertrichosis
- •Be aware that subtle hypertrichosis or absent clinical signs do not rule out emerging PPID; integrate seasonal testing patterns and stimulation protocols into your diagnostic approach for early identification
Key Findings
- •Two horses developed marked hypertrichosis with basal ACTH concentrations progressing from equivocal to consistently above diagnostic cut-off values without treatment
- •TRH-stimulated ACTH testing was more sensitive than basal ACTH, detecting elevated concentrations in 57.9% of testing points in transitional horses versus 47.4% for basal measurements
- •TRH-stimulated ACTH concentrations showed strong seasonal variation with highest detection rates in late summer and early autumn (February-March), with 64.9% exceeding diagnostic cut-off values
- •Horses transitioning to PPID can present with subtle or absent clinical signs and equivocal basal ACTH concentrations, making early diagnosis challenging without sequential testing and TRH stimulation