A randomised, controlled trial to determine the effect of levothyroxine on Standardbred racehorses

Diagnosis and management of thyroid disorders and thyroid hormone supplementation in adult horses and foals

Equine thyroid disorders pose a diagnostic challenge in clinical practice because of the effects of nonthyroidal factors on the hypothalamic-pituitary-thyroid axis, and the horse's ability to tolerate wide fluctuations in thyroid hormone concentrations and survive without a thyroid gland. While benign thyroid tumours are common in older horses, other disorders like primary hypothyroidism or hyperthyroidism in adult horses and congenital hypothyroidism in foals are rare. There is a common misunderstanding regarding hypothyroidism in adult horses, especially when associated with the clinical profile of obesity, lethargy, and poor performance observed in dogs and humans. Low blood thyroid hormone concentrations are often detected in horses as a secondary response to metabolic and disease states, including with the nonthyroidal illness syndrome; however, it is important to note that low thyroid hormone concentrations in these cases do not necessarily indicate hypothyroidism. Assessing equine thyroid function involves measuring thyroid hormone concentrations, including total and free fractions of thyroxine (T4) and triiodothyronine (T3); however, interpreting these results can be challenging due to the pulsatile secretion of thyroid hormones and the many factors that can affect their concentrations. Dynamic testing, such as the thyrotropin-releasing hormone stimulation test, can help assess the thyroid gland response to stimulation. Although true hypothyroidism is extremely rare, thyroid hormone supplementation is commonly used in equine practice to help manage obesity and poor performance. This review focuses on thyroid gland pathophysiology in adult horses and foals, interpretation of blood thyroid hormone concentrations, and evaluation of horses with thyroid disorders. It also discusses the use of T4 supplementation in equine practice.

A one-health lens offers new perspectives on the importance of endocrine disorders in the equine athlete

Endocrine disorders are associated with joint pain and tendon injury in humans, but the effects in the horse are only starting to be understood. Similar patterns of clinical signs and injury appear to affect horses and humans for both orthopedic and endocrine disorders, supporting the use of a one-health approach to tackle these issues. In this Currents in One Health, we will discuss common equine endocrinopathies, current testing recommendations, dietary management, genetic predispositions, and endocrine disorders' effects on performance. Our aim is to use a one-health lens to describe current comparative research so that veterinarians can employ cutting-edge preventative, diagnostic, and therapeutic recommendations. Identified key gaps in knowledge include whether equine metabolic osteoarthritis exists, if steroid joint injections are safe in horses with endocrine disorders, and if the return to performance percentage improves with concurrent treatment of endocrine and musculoskeletal disorders. Key takeaways include that the relationship between endocrine disorders and musculoskeletal disease in the horse goes beyond laminitis to include lameness, muscle atrophy, suspensory ligament degeneration, osteochondritis dissecans, and potentially metabolic osteoarthritis. Approaches learned from human and equine comparative studies can offer insight into injury recognition and management, thus mitigating the impact of endocrine disorders on performance in both species. Readers interested in an in-depth description of current and future research involving pathophysiology, novel interventions, and multiomic approaches to identify individuals with athletic limitations induced by endocrine disorders are invited to read the companion Currents in One Health by Manfredi et al, AJVR, February 2023.

A one-health approach to identifying and mitigating the impact of endocrine disorders on human and equine athletes

Endocrinopathies affect multiple species in ever-increasing percentages of their populations, creating an opportunity to apply one-health approaches to determining creative preventative measures and therapies in athletes. Obesity and alterations in insulin and glucose dynamics are medical concerns that play a role in whole-body health and homeostasis in both horses and humans. The role and impact of endocrine disorders on the musculoskeletal, cardiovascular, and reproductive systems are of particular interest to the athlete. Elucidation of both physiologic and pathophysiologic mechanisms involved in disease processes, starting in utero, is important for development of prevention and treatment strategies for the health and well-being of all species. This review focuses on the unrecognized effects of endocrine disorders associated with the origins of metabolic disease; inflammation at the intersection of endocrine disease and related diseases in the musculoskeletal, cardiovascular, and reproductive systems; novel interventions; and diagnostics that are informed via multiomic and one-health approaches. Readers interested in further details on specific equine performance conditions associated with endocrine disease are invited to read the companion Currents in One Health by Manfredi et al, JAVMA, February 2023.