Circulating Hypothalamic-Pituitary-Adrenal Axis Hormones and Insulin Concentrations in Horses and Ponies.
Authors: Vaughn Sarah A, Norton Natalie A, Hart Kelsey A
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Circulating HPA Axis Hormones and Insulin in Horses and Ponies Ponies are notoriously prone to insulin dysregulation (ID), yet the underlying mechanisms remain unclear—a knowledge gap this 2022 study addressed by investigating whether breed-related differences in cortisol metabolism might explain this predisposition. The researchers measured plasma ACTH, insulin, total serum cortisol, and the free cortisol fraction (FCF—the biologically active, unbound hormone) in 33 horses and 24 ponies, both healthy and with ID, hypothesising that ponies would exhibit elevated FCF that correlated with hyperinsulinaemia. Whilst healthy animals showed comparable total cortisol and insulin between breeds, ponies displayed 29% higher ACTH concentrations (P = 0.016), suggesting differential HPA axis activity; notably, in ID cases, FCF increased 40% in ponies relative to horses despite similar total cortisol and insulin levels, pointing to a pony-specific shift in cortisol-binding dynamics. These findings suggest that enhanced free cortisol availability—rather than absolute cortisol or insulin quantities—may contribute to ponies' heightened susceptibility to ID, potentially through cortisol's antagonistic effects on insulin signalling. For practitioners managing obese or dysregulated ponies, this research emphasises that assessing HPA axis function and stress-related cortisol metabolism may warrant as much attention as direct insulin measurement, and warrants further investigation into whether targeted stress management or HPA-modulating interventions could mitigate ID risk in this high-risk population.
Read the full abstract on PubMed
Practical Takeaways
- •Ponies with insulin dysregulation have disproportionately elevated biologically active cortisol relative to total cortisol, suggesting breed-specific hormonal mechanisms that may require different monitoring or management strategies
- •Baseline ACTH is naturally higher in healthy ponies, so reference ranges should not be directly compared between horses and ponies when evaluating HPA axis function
- •Assessment of insulin dysregulation risk in ponies should consider that breed-related hormonal differences may amplify cortisol's antagonistic effects on insulin signaling
Key Findings
- •ACTH concentrations were 29% higher in healthy ponies compared to horses (P=0.016)
- •In insulin dysregulation, free cortisol fraction was increased 40% in ponies compared to horses despite similar total cortisol and insulin concentrations
- •Total cortisol, free cortisol, insulin, and free cortisol fraction were comparable between healthy horses and ponies
- •Breed-related differences in free cortisol fraction may contribute to pony predisposition to insulin dysregulation