Seasonal Variation in Diagnostic Tests for Pituitary Pars Intermedia Dysfunction in Normal Aged Geldings
The purpose of the study was to determine if seasonal variations exist in Alabama in the results of various tests used to diagnose pituitary pars intermedia dysfunction (PPID) in normal horses. In addition, we aimed to provide reference ranges for endogenous plasma adrenocorticotrophin (ACTH), α-melanocyte-stimulating hormone (α-MSH), and serum insulin and cortisol concentrations, and cortisol concentrations 15 and 19hrs after a dexamethasone suppression test (DST) for each season of the year.
Fifteen healthy mixed breed geldings aged 12-25 years were used. Baseline blood samples were collected at 4 p.m. for determination of endogenous ACTH, α-MSH, cortisol and insulin concentrations. Dexamethasone (40µg/kg) was then injected intramuscularly (T=0). Blood was collected at T=15 hrs (7am) and T=19 hrs (11am) for determination of cortisol concentrations. Sample collection was repeated monthly for 12 months. Radioimmunoassays were used to measure all hormones. Differences in hormone concentrations by season and month were assessed using repeated measures ANOVA; P<0.05 was considered significant.
There was a significant effect of season on α-MSH (P=0.0019) concentrations and cortisol response to DST at T=19hrs (P=0.02). There was a tendency toward a seasonal effect on ACTH (P=0.07), but no effect on insulin or cortisol at T=0 or T=15hr. There was no significant time effect in any variable when analyzed by month.
The seasonal effect on tests for PPID appears to be less marked in Alabama compared to the northeastern United States, and insufficient to affect the validity of tests evaluating ACTH, insulin or cortisol concentrations in most horses. Seasonal reference ranges should be used for α-MSH.