Thyroid Disease in Horses—Retrospective Case Series on Patients Examined for Thyroid Disease in an Equine University Clinic (2009–2024)

Thyroid diseases in horses are rare and pose challenges for veterinarians due to the complexity of clinical and diagnostic approaches. This retrospective case series describes cases of equine thyroid disease in a referral hospital population. Files of horses examined for suspected thyroid disease at an equine university clinic between 2009 and 2024 were reviewed. Data from nine horses examined for suspected thyroid disease, including signalment, clinical presentation, laboratory results, imaging findings, fine needle aspirates, biopsies, diagnoses, and treatments, were summarised. Diagnoses included thyroid adenoma (<i>n</i> = 6), multiple thyroid cysts (possibly thyroid adenoma (<i>n</i> = 1)), non-thyroidal illness syndrome (NTIS, <i>n</i> = 1), and iodine deficiency goitre (<i>n</i> = 1). Diagnostic modalities varied, with, besides manual palpation performed in every patient, basal thyroid hormone measurement and ultrasonography being the most common. Low serum iodine concentrations were noted in four horses, yet iodine supplementation was inconsistently applied. Hemithyroidectomy was performed in four horses, enabling histopathological diagnosis (three out of four). Despite being rare, thyroid diseases in horses require consistent and thorough diagnostic approaches, including imaging, laboratory, and histopathological examinations. Improved screening tools and research could enhance the diagnostic accuracy and management of equine thyroid disorders.