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The Journal of veterinary medical science2025; 87(7); 804-815; doi: 10.1292/jvms.25-0061

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

Abstract: 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.
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Publication Date: 2025-05-23 PubMed ID: 40414721PubMed Central: PMC12246564DOI: 10.1292/jvms.25-0061Google Scholar: Lookup
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  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Overview

  • This study investigated how the month of birth influences the hormonal development of Thoroughbred foals born in Hokkaido, Japan, focusing on endocrine functions from birth to 24 weeks of age.
  • It examined key hormones related to growth and adaptation to environmental factors, to understand whether early-born foals experience any endocrine disadvantages due to the cold winter climate.

Introduction and Purpose

  • Thoroughbred foals born earlier in the year are generally thought to have a competitive advantage in racing due to their advanced development.
  • Hokkaido, located in northern Japan, has a harsh cold winter climate that could potentially affect the foals’ physiological development, especially their endocrine function.
  • The research aimed to clarify how the month of birth (from February to May) affects the development of three major endocrine systems in foals: the hypothalamic-pituitary axis, adrenal function, and thyroid function.
  • Specifically, the study compared hormone levels related to these systems over the first 24 weeks of life in foals born at different times within this early-year window.

Methodology

  • The study population consisted of 87 Thoroughbred foals born in Hokkaido between February and May.
  • Foals were grouped based on their birth month to compare hormone changes over time.
  • Hormones measured included:
    • Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): indicators of hypothalamic-pituitary axis activity.
    • Prolactin: associated with responses to day length and various physiological processes.
    • Cortisol: reflecting adrenal stress hormone activity.
    • Thyroxine (T4): representing thyroid function important for metabolism and growth.
  • Hormone measurements were performed at multiple time points from birth until 24 weeks of age.
  • Comparisons were made for each sex to determine whether hormonal development differed between males and females or based on birth month.

Key Findings

  • Hypothalamic-Pituitary Axis Development:
    • LH and FSH secretion increased in response to prolonged day lengths starting from about 10 weeks of age.
    • This response occurred similarly regardless of birth month or sex, indicating normal maturation of this axis during early postnatal life.
  • Prolactin Concentrations:
    • An increase in prolactin levels from May to August was observed, suggesting that foals can respond to changes in day length from a very early age.
    • This response was independent of physical growth, indicating an innate sensitivity to environmental light cues soon after birth.
  • Adrenal (Cortisol) and Thyroid (Thyroxine) Functions:
    • At birth, cortisol and thyroxine levels were elevated irrespective of the foal’s birth month.
    • This suggests that these endocrine systems are fully activated by late gestation to support rapid growth and to help the newborn adapt to the external environment.
    • These hormone levels were within normal ranges even in early-born foals, indicating that the cold Hokkaido winter did not negatively affect their adrenal and thyroid development.

Conclusions and Implications

  • Endocrine functions related to growth and environmental adaptation develop appropriately in Thoroughbred foals born from February to May in Hokkaido, despite the harsh winter conditions.
  • Early birth does not appear to compromise the hormonal development necessary for growth and maturation.
  • Foals show the ability to respond to environmental cues such as day length soon after birth, which is important for seasonal physiological regulation.
  • Findings support the idea that early-born foals can maintain normal endocrine function, potentially retaining the racing advantage previously attributed to their earlier birth date without endocrine disadvantage due to climate.

Cite This Article

APA
Ishimaru M, Kume K, Murase H, Sato F, Matsui A, Ohmura H, Taya K. (2025). Effect of birth month on endocrine function in Thoroughbred foals born in Hokkaido, the northern part of Japan. J Vet Med Sci, 87(7), 804-815. https://doi.org/10.1292/jvms.25-0061

Publication

The Journal of veterinary medical science
ISSN: 1347-7439
NlmUniqueID: 9105360
Country: Japan
Language: English
Volume: 87
Issue: 7
Pages: 804-815

Researcher Affiliations

Ishimaru, Mutsuki
  • International Department, Japan Racing Association, Tokyo, Japan.
Kume, Koichi
  • Equine Department, Japan Racing Association, Tokyo, Japan.
Murase, Harutaka
  • Equine Department, Japan Racing Association, Tokyo, Japan.
Sato, Fumio
  • Japan Farriery Association, Tokyo, Japan.
Matsui, Akira
  • Hidaka Training and Research Center, Japan Racing Association, Hokkaido, Japan.
Ohmura, Hajime
  • Hidaka Training and Research Center, Japan Racing Association, Hokkaido, Japan.
Taya, Kazuyoshi
  • Laboratory of Veterinary Physiology, Cooperative Department of Veterinary Medicine, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo, Japan.

MeSH Terms

  • Animals
  • Female
  • Male
  • Animals, Newborn / physiology
  • Follicle Stimulating Hormone / blood
  • Horses / physiology
  • Horses / growth & development
  • Horses / blood
  • Hydrocortisone / blood
  • Hypothalamo-Hypophyseal System / physiology
  • Japan
  • Luteinizing Hormone / blood
  • Prolactin / blood
  • Seasons
  • Thyroxine / blood

Conflict of Interest Statement

The authors declare no conflicts of interest associated with this manuscript.

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