Circannual variation in plasma adrenocorticotropic hormone concentrations in the UK in normal horses and ponies, and those with pituitary pars intermedia dysfunction.
Authors: Copas V E N, Durham A E
Journal: Equine veterinary journal
Summary
# Circannual Variation in Equine Plasma ACTH: Implications for PPID Diagnosis Pituitary pars intermedia dysfunction (PPID) represents a significant endocrinopathy in horses and ponies, yet diagnosis relies heavily on plasma adrenocorticotropic hormone (ACTH) concentration thresholds that may not account for natural seasonal fluctuations. Copas and Durham examined monthly plasma ACTH samples from healthy horses and ponies alongside PPID-affected animals across a full year in the UK, establishing how reference intervals vary seasonally and how this variation manifests in diseased animals. Their findings revealed marked circannual rhythm in ACTH concentrations in both normal and PPID populations, with distinct seasonal peaks that could substantially affect diagnostic accuracy if standard year-round reference ranges were applied. For equine practitioners, this work underscores the critical importance of contextualising ACTH results within the season of sampling—diagnosis based on autumn or winter samples risks misclassification, whilst spring and summer results may offer greater diagnostic reliability. These findings should inform evidence-based diagnostic protocols and highlight the necessity for season-specific reference intervals when interpreting ACTH results for suspected PPID cases.
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Practical Takeaways
- •When interpreting ACTH results for PPID diagnosis, consider the time of year—elevated ACTH may reflect normal seasonal variation rather than disease
- •Use season-appropriate reference intervals for plasma ACTH rather than single annual cutoff values to improve diagnostic accuracy
- •Clinicians should exercise caution diagnosing PPID during periods of naturally elevated ACTH (typically autumn months in Northern Hemisphere)
Key Findings
- •Plasma ACTH concentrations in normal horses and ponies show significant circannual variation, with seasonal peaks affecting diagnostic reference intervals
- •Seasonal variation in ACTH must be considered when diagnosing PPID to avoid misclassification at certain times of the year
- •Appropriate reference intervals for equine plasma ACTH vary throughout the year and were established for UK populations