Effect of birth month on endocrine function in Thoroughbred foals born in Hokkaido, the northern part of Japan.

Thoroughbreds born early in the year are believed to have an advantage in racing. However, the development of endocrine functions in early-born foals in the cold winter climate is unclear. The present study was conducted to elucidate the effect of birth month on the development of endocrine function in Thoroughbred foals born from February to May in Hokkaido by comparing hypothalamic-pituitary axis, adrenal and thyroid functions from birth to 24 weeks of age. Eighty-seven Thoroughbred foals were used to compare the changes in circulating luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, cortisol, and thyroxine among four birth month groups for each sex. The present study revealed that as foals grow, the hypothalamic-pituitary axis develops, allowing them to secrete LH and FSH in response to prolonged day length from about 10 weeks of age, regardless of birth month for both sexes. In addition, the increase in prolactin concentrations from May to August suggests that postnatal foals may have the ability to respond to prolonged daylength immediately after birth, independent of growth. Furthermore, circulating cortisol and thyroxine in the foals at birth were elevated regardless of birth month, suggesting that adrenal and thyroid functions were fully activated for the rapid growth during late gestation and adaptation to the external environment that occurs early postnatal period. In early-born foals, these hormone concentrations were found to be within the normal range, indicating that development of foals were not affected by the harsh Hokkaido winter.