Investigation of rhythms of secretion and repeatability of plasma adrenocorticotropic hormone concentrations in healthy horses and horses with pituitary pars intermedia dysfunction.
Authors: Rendle D I, Litchfield E, Heller J, Hughes K J
Journal: Equine veterinary journal
Summary
# Editorial Summary: ACTH Measurement Reliability in PPID Diagnosis Rendle and colleagues investigated whether daily and hourly fluctuations in ACTH secretion affect the reliability of single-point plasma ACTH testing for diagnosing pituitary pars intermedia dysfunction. Over three weeks, they measured ACTH concentrations at four timepoints daily across five non-consecutive days in eight healthy horses and eight PPID-affected horses, supplemented by rapid serial sampling at five-minute intervals to detect short-term variations. Whilst no ultradian (hourly) rhythms were evident in either group, healthy horses displayed a modest circadian pattern with highest concentrations at 08:00 h that declined through the day—though this variation was smaller than random fluctuations between individual measurements. Critically, PPID horses lacked this circadian pattern entirely and exhibited substantially greater day-to-day variability in ACTH concentration than healthy animals, suggesting inherent biological inconsistency in their pituitary hormone output. These findings indicate that clinicians encountering borderline ACTH results (19–40 pg/ml) should pursue confirmatory testing rather than relying on a single measurement, particularly given the increased unpredictability of ACTH secretion in PPID-affected horses; the timing of sampling appears less influential than individual horse factors yet to be elucidated.
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Practical Takeaways
- •Single timepoint ACTH measurements are reliable for diagnosis in non-PPID horses, but PPID horses show greater unpredictable variation requiring repeat testing
- •Timing of sample collection (morning vs afternoon) is unlikely to significantly affect interpretation in healthy horses, but intermediate ACTH values (19-40 pg/ml) warrant additional diagnostic investigation
- •PPID horses demonstrate increasingly variable ACTH levels as disease progresses, supporting the need for serial measurements rather than relying on one-off results in borderline cases
Key Findings
- •No evidence of ultradian fluctuations in plasma ACTH concentration in either healthy or PPID horses
- •Circadian rhythm detected in non-PPID horses with highest ACTH at 08:00 h, but not in PPID horses
- •Intrahorse variability of ACTH concentration was significantly greater in PPID horses than non-PPID horses
- •When ACTH concentration measures 19-40 pg/ml, further testing should be considered to improve diagnostic accuracy for PPID