Ponies have different ACTH reference numbers in the fall

Normal ponies were found to have significantly higher ACTH serum concentrations in the Fall compared to TB horses. The differences are high enough to affect interpretation. Surprisingly this was not true of a Spring test.
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Investigation of breed differences in plasma adrenocorticotropic hormone concentrations among healthy horses and ponies
Vet J. 2023 May 17;105995. doi: 10.1016/j.tvjl.2023.105995. Online ahead of print.
Authors
N J Bamford 1 , A J Stewart 2 , C M El-Hage 3 , F R Bertin 2 , S R Bailey 3
Affiliations

1 Melbourne Veterinary School, The Universtiy of Melbourne, Building 400, Parkville, Victoria 3010 Australia. Electronic address: n.bamford@unimelb.edu.au.
2 School of Veterinary Science, The University of Queensland, Building 4114, Gatton, Queensland 4343 Australia.
3 Melbourne Veterinary School, The Universtiy of Melbourne, Building 400, Parkville, Victoria 3010 Australia.

PMID: 37207985
DOI: 10.1016/j.tvjl.2023.105995

Abstract

Plasma adrenocorticotropic hormone (ACTH) concentration is commonly measured to diagnose pituitary pars intermedia dysfunction (PPID). Several intrinsic and extrinsic factors affect ACTH concentrations, including breed. The objective of this study was to prospectively compare plasma ACTH concentrations among different breeds of mature horses and ponies. Three breed groups comprised Thoroughbred horses (n=127), Shetland ponies (n=131) and ponies of non-Shetland breeds (n=141). Enrolled animals did not show any signs of illness, lameness or clinical signs consistent with PPID. Blood samples were collected 6 months apart, around the autumn equinox and spring equinox, and plasma concentrations of ACTH were measured by chemiluminescent immunoassay. Pairwise breed comparisons within each season were performed on log transformed data using the Tukey test. Estimated mean differences in ACTH concentrations were expressed as fold difference with 95% confidence interval. Reference intervals for each breed group per season were calculated using non-parametric methods. In autumn, higher ACTH concentrations were found among non-Shetland pony breeds compared with Thoroughbreds (1.55 fold higher; 95% confidence intervals [CI], 1.35-1.77; P<0.001), and in Shetland ponies compared with Thoroughbreds (2.67 fold higher; 95% CI, 2.33-3.08; P<0.001) and non-Shetland pony breeds (1.73 fold higher; 95% CI, 1.51-1.98; P<0.001). In spring, no differences were identified among breed groups (all P>0.05). Reference intervals were similar among breed groups in spring, but upper limits for ACTH concentrations were markedly different between Thoroughbred horses and pony breeds in autumn. These findings emphasise that breed should be accounted for when determining and interpreting reference intervals for ACTH concentrations among healthy horses and ponies in autumn.

Keywords: ACTH; Endocrine; Equine; Pituitary pars intermedia dysfunction; Reference interval.