Adrenocorticotropic Hormone and Thyroxine Concentrations in Young Healthy Equids: Species Differences and Seasonal Variations

Due to growing interest in endocrine diseases, more studies aim to interpret the baseline concentrations and reference intervals of hormone values like adrenocorticotropic hormone (ACTH) and thyroxine (T4) in horses. Most studies available in the literature focus on evaluating ACTH and thyroid hormone values in adult and geriatric horses, with only a few considering younger donkeys and ponies. This study aimed to measure ACTH and T4 in autumn and spring in younger equids. The study included 42 young equids, which were divided into three groups: horses, ponies, and donkeys. The concentration of T4 was significantly higher in autumn than in spring in all the animals (p = 0.049) and significantly higher in donkeys than in horses (p = 0.0046). The concentration of ACTH was significantly higher in autumn than in spring in all animals (p = 0.00015) and significantly higher in donkeys than in horses and ponies (p = 0.001). The study results underscore the importance of utilizing seasonal hormone reference ranges specifically adapted for ponies and donkeys to accurately assess hormone secretion patterns in young animals. Reliance on reference values established for horses may lead to overinterpretation of hormone concentrations and misdiagnosis of endocrine disease in healthy donkeys or ponies.