Hypothalamic-pituitary-adrenal axis assessment in healthy term neonatal foals utilizing a paired low dose/high dose ACTH stimulation test.
Authors: Hart K A, Heusner G L, Norton N A, Barton M H
Journal: Journal of veterinary internal medicine
Summary
# Hypothalamic-pituitary-adrenal axis assessment in healthy neonatal foals The HPA axis undergoes rapid maturation during the first week of life, yet reference ranges for HPA function in neonatal foals remain poorly defined—a gap this 2009 study addressed by establishing age-specific cortisol and ACTH responses using a paired low/high dose cosyntropin stimulation protocol in 20 healthy foals assessed at birth, 12–24 hours, 36–48 hours, and 5–7 days of age. Foals at birth demonstrated significantly elevated basal cortisol and ACTH concentrations (with higher ACTH:cortisol ratios) compared to all subsequent time points, indicating the profound neuroendocrine stress of parturition; both the 10 and 100 microgram cosyntropin doses elicited marked cortisol responses across all age groups, though the magnitude of response was substantially greater in younger foals and declined significantly with age. The paired stimulation protocol itself did not confound results, making it suitable for comprehensive HPA axis evaluation in neonates, though interpretation of cortisol values is critically dependent on foal age. For practitioners managing critically ill or septic neonatal foals, recognising that age-dependent cortisol responses are physiologically normal rather than pathological is essential; inappropriately low cortisol values may warrant investigation, but reference ranges must account for whether the foal is hours or days old to avoid misdiagnosis and unnecessary intervention.
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Practical Takeaways
- •Age-specific reference ranges are essential when interpreting HPA axis function tests in neonatal foals, as cortisol and ACTH values and responses change dramatically within the first week of life
- •The paired 10/100 microg cosyntropin stimulation test is a reliable diagnostic tool for assessing HPA axis function in foals of all neonatal ages
- •Basal cortisol and ACTH values are highest at birth and decline progressively, so baseline values should be interpreted relative to foal age
Key Findings
- •Foals at birth had significantly higher basal cortisol and ACTH concentrations compared to foals at 12-24 hours, 36-48 hours, and 5-7 days of age
- •Cortisol response to both 10 and 100 microg cosyntropin doses was significantly greater in younger foals and decreased with age
- •Paired low dose/high dose cosyntropin stimulation test can be successfully used to evaluate HPA axis function in neonatal foals
- •Test design itself had no effect on cortisol responses, validating the paired protocol for clinical use