Objective: To determine whether seasonal variations exist in endogenous plasma ACTH, plasma α-melanocyte-stimulating hormone (α-MSH), serum cortisol, and serum insulin concentrations and in the results of a dexamethasone suppression test for older, clinically normal geldings in Alabama.
Design: Cohort study.
Animals: 15 healthy mixed-breed geldings (median age, 14 years).
Procedures: Sample collection was repeated monthly for 12 months. Dexamethasone (0.04 mg/kg [0.02 mg/lb], IM) was administered and cortisol concentrations were determined at 15 and 19 hours. Radioimmunoassays were used to measure ACTH, α-MSH, cortisol, and insulin concentrations at each testing time. Hormone concentrations were compared between months via repeated-measures ANOVA and correlated with age within each month.
Results: A significant time effect was found between months for α-MSH and insulin concentrations. Endogenous cortisol and ACTH concentrations remained within existing reference ranges. Significant correlations were detected between age and ACTH concentration for several fall and winter months and between age and insulin concentration for September.
Conclusions and clinical relevance: Older horses have higher ACTH concentrations in several fall and winter months and higher insulin concentrations in September than do younger horses. Seasonally specific reference ranges are required for α-MSH and insulin concentrations, with significantly higher concentrations detected in the fall. Practitioners should be advised to submit samples only to local laboratories that can provide such reference ranges for their local geographic region.