The use of adrenocorticotrophic hormone as a potential biomarker of pituitary pars intermedia dysfunction in horses

Elevated concentrations of plasma adrenocorticotrophic hormone (ACTH) in horses with pituitary disturbances are often associated with dysfunction of the pituitary’s pars intermedia. The majority of such animals exhibit an increased susceptibility to laminitis, particularly during the autumn. The 24 h plasma ACTH profiles of horses with pituitary pars intermedia dysfunction (PPID), and those of matched controls, were determined in March, June, September and December. Differences in ACTH concentrations between the groups were significant (P < 0.0001), regardless of photoperiod, and persisted throughout the 24 h cycle in all seasons. ACTH concentrations were significantly higher in September and December than in March and June in both groups of animals. Cosinor analysis of the data indicated that the amplitude of changes was season-independent but higher (P = 0.0441) in PPID horses and that a significant interaction (P = 0.0096) existed between body condition (control versus PPID) and season of peak ACTH response. There were significant, but opposite, phase-shift differences in June and September. These results suggest that ACTH has a role as a biomarker of equine PPID providing that appropriate ‘cut-off’ values are used at different times of the year.