Evaluation of basal plasma α-melanocyte-stimulating hormone and adrenocorticotrophic hormone concentrations for the diagnosis of pituitary pars intermedia dysfunction from a population of aged horses.
Authors: Mc Gowan T W, Pinchbeck G P, Mc Gowan C M
Journal: Equine veterinary journal
Summary
# Basal Hormone Testing for Equine PPID: Seasonal Timing Matters Pituitary pars intermedia dysfunction (PPID) affects many aged horses, yet reliable diagnostic markers remain inconsistently applied in practice. This population-based study examined whether measuring basal plasma α-melanocyte-stimulating hormone (α-MSH) and adrenocorticotrophic hormone (ACTH) could accurately diagnose PPID in horses aged 15 years and older, establishing clinically useful cut-off values derived from owner questionnaires and veterinary examination of 82 horses against a gold standard diagnosis (hirsutism plus ≥3 clinical signs). Both hormones showed strong correlation with each other and clinical disease severity, with α-MSH at 52.0 pmol/l and ACTH at 29.7 pg/ml providing moderate-to-good diagnostic accuracy outside autumn months (59% and 80% sensitivity respectively; 93% and 83% specificity), though notably improving to near-perfect performance during the autumn period. The marked seasonal effect—with higher cut-off thresholds needed in autumn and sensitivity/specificity approaching 100%—suggests practitioners should preferentially sample during non-autumn months to optimise diagnostic yield, whilst acknowledging that location-specific reference intervals and seasonal adjustments will likely refine interpretation further. For equine professionals managing older horses suspected of PPID, these findings underpin why single baseline hormone measurements require contextual interpretation and why autumn sampling may paradoxically be most diagnostically valuable despite higher reference ranges.
Read the full abstract on PubMed
Practical Takeaways
- •Schedule PPID screening in autumn when basal α-MSH and ACTH testing shows optimal diagnostic performance (near-perfect sensitivity and specificity)
- •If testing outside autumn, use seasonally adjusted cut-off values (α-MSH ≥52.0 pmol/l, ACTH ≥29.7 pg/ml) and expect moderate sensitivity—consider repeat testing or supplementary diagnostics if clinical suspicion remains high
- •Both biomarkers are useful for confirming PPID in aged horses (≥15 years) presenting with hirsutism and multiple clinical signs, but autumn testing provides the most reliable results
Key Findings
- •α-MSH and ACTH showed moderate-to-good sensitivity and specificity for PPID diagnosis in non-autumn seasons (59% and 93% for α-MSH; 80% and 83% for ACTH respectively)
- •Both biomarkers demonstrated near 100% sensitivity and specificity during autumn sampling, suggesting seasonal variation significantly impacts diagnostic accuracy
- •α-MSH and ACTH concentrations correlated strongly with each other and with clinical signs, supporting a spectrum of disease severity in PPID