Variation in plasma adrenocorticotropic hormone concentration and dexamethasone suppression test results with season, age, and sex in healthy ponies and horses.
Authors: Donaldson Mark T, McDonnell Sue M, Schanbacher Barbara J, Lamb Stephen V, McFarlane Dianne, Beech Jill
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary Distinguishing equine pituitary pars intermedia dysfunction (PPID) from normal seasonal physiology remains clinically challenging, yet this 2005 study provides crucial reference data for interpreting ACTH and dexamethasone suppression test (DST) results across different seasons and populations. Researchers measured plasma ACTH concentrations in 15 pony mares, 14 pony stallions, and 10 horse mares living outdoors across four timepoints (September 2002, January, May, and September 2003), and performed DSTs in January and September 2003 to establish how these markers fluctuate in healthy animals. The findings revealed substantial seasonal variation: ACTH concentrations in September were significantly elevated compared to winter and spring (P <0.001), with only 5–8% of healthy animals showing ACTH levels within standard reference ranges in autumn, compared to 97–100% in winter and spring; similarly, DST suppression failed in 26% of healthy subjects in September but in none during January. Older animals showed higher ACTH and cortisol concentrations regardless of season, whilst sex and species (ponies versus horses) had no significant effect on these values. These findings underscore that interpreting elevated ACTH or impaired DST results requires careful consideration of sampling month and age; practitioners should establish baseline measurements during winter months when possible, avoid overdiagnosing PPID based on autumn sampling alone, and recognise that autumn elevation of ACTH within otherwise healthy animals represents normal seasonal physiology rather than pathology.
Read the full abstract on PubMed
Practical Takeaways
- •Interpret ACTH and DST results with consideration of season—elevated values in late summer/early fall may be normal physiologic variation rather than pathologic disease in healthy animals
- •Reference ranges for endocrine testing in equines should account for seasonal variation; autumn testing shows higher baseline ACTH and reduced DST suppression compared to winter/spring
- •Age influences baseline ACTH and cortisol responses; older animals tend to have higher concentrations even when healthy, which should inform clinical interpretation
Key Findings
- •Plasma ACTH concentrations were significantly higher in September (5-8% within reference range) compared to January and May (97-100% within reference range), P < .001
- •DST results were within reference range for all subjects in January but only 74% of subjects in September 2003
- •Age was positively correlated with plasma ACTH and cortisol concentrations at baseline and post-DST
- •No significant differences in ACTH concentration between pony mares, pony stallions, and horse mares within the same season (P > .05)