The Influence of Season on Endocrine Testing in Equids.

Endocrine function in equids is strongly influenced by season, with photoperiod as the primary driver and additional modulation by factors such as breed, geography, age, and nutrition. These seasonal effects have important implications for the interpretation of endocrine diagnostic tests. Circannual rhythms influence pituitary activity, resulting in predictable seasonal variation in adrenocorticotropic hormone concentrations and forming the basis for seasonally adjusted diagnostic thresholds for pituitary pars intermedia dysfunction. Seasonal effects on insulin dynamics are less understood but increasingly support a winter-associated increase in serum concentrations. Recognizing seasonal patterns improves the interpretation of endocrine test results and supports accurate clinical decision-making.