A hormone is a chemical messenger synthesized by an endocrine gland and secreted directly into the bloodstream, where it travels to target organs and regulates their activity. The term derives from the Greek word meaning “to set in motion,” accurately capturing how tiny quantities of a hormone can trigger or suppress major physiological processes far from the site of production. In horses, the endocrine system governs reproductive cycling, pregnancy maintenance, growth, metabolism, stress adaptation, and fluid balance through an interlocking network of hormones produced by the pituitary, thyroid, adrenal, and gonadal glands, among others.
Reproductive hormones are among the most clinically significant in equine management. Estrogen and progesterone produced by the ovaries regulate the breeding mare's estrous cycle; progesterone is administered exogenously to suppress cycling, synchronize mares for breeding, or maintain pregnancy in mares with luteal insufficiency. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH), released by the anterior pituitary, control follicular development and ovulation. Human chorionic gonadotropin (hCG) and deslorelin acetate are synthetic compounds used to induce ovulation by mimicking LH. In the intact male, testosterone produced by the testes drives libido, sperm production, and secondary sex characteristics; the absence of these testosterone effects after castration produces the behavioral change sought in a gelding.
Cortisol, the primary glucocorticoid secreted by the adrenal cortex in response to adrenocorticotropic hormone (ACTH), mediates the stress response and suppresses systemic inflammatory response. Pituitary pars intermedia dysfunction (PPID, commonly called equine Cushing's disease) results from dysregulated ACTH and elevated cortisol, producing clinical signs that include a long, wavy coat, laminitis, and increased water intake. Insulin dysregulation, in which tissues lose sensitivity to insulin—the pancreatic hormone that governs blood glucose uptake—is the central mechanism behind equine metabolic syndrome and the resulting risk of laminitis. Thyroid hormones (T3 and T4) regulate basal metabolic rate and are occasionally assessed in horses with unexplained weight loss or lethargy, though primary hypothyroidism is rare in adults.
Further Reading
- hormones (Wikipedia)
- For clinical context on hormonal imbalances and metabolic disease: metabolic disorders and endocrine function in horses — MSD Veterinary Manual