Spring and fall blood concentrations of adrenocorticotropic hormone, insulin, and thyroxine in healthy horses in Saskatchewan.

Objective: To characterize concentrations of adrenocorticotropic hormone (ACTH), insulin, and thyroxine (T4) at 2 times of the year in healthy horses in Saskatchewan.

Animals And Procedure: A prospective, observational study was carried out in 52 healthy, client-owned horses. Inclusion criteria were no recent travel outside of Saskatchewan, normal physical examination findings, and no history or evidence of ongoing illness. Blood concentrations of ACTH, insulin and T4 were determined by chemiluminescence. Samples were collected in spring and fall and compared using the paired Student's -test or Wilcoxon signed-rank test. Generalized estimating equations were used to assess the associations between ACTH, T4, and insulin concentrations and gender, age, season, body condition score, glucose concentration, and breed.

Results: There were increases in both ACTH and insulin concentrations in the fall compared to spring ( < 0.001 and = 0.001, respectively). Other than season, insulin concentration was associated with breed, whereas ACTH concentration was associated with age. Finally, T4 concentration was associated with breed and glucose concentration, but not with season.

Conclusion And Clinical Relevance: These results highlighted the differences between spring and fall concentrations of both ACTH and insulin in healthy horses residing in the Canadian prairie provinces, which are known for extreme differences in summer and winter temperatures as well as day length. Geographically adjusted reference values are necessary to account for these variations, to improve diagnostic accuracy. This is the first published Canadian study evaluating these factors and their associations with hormone concentrations in clinically healthy animals.