Mediastinal Ectopic Thyroid Carcinoma and Concurrent Multinodular Pulmonary Fibrosis in a Horse

A 30-year-old Standardbred gelding was referred for chronic pleural effusion. Thoracic ultrasound revealed marked bilateral pleural effusion and a large heterogeneous mass within the cranial mediastinum, which extended from the right 5th to the 11th intercostal space. Subsequently, on thoracic radiographs, the cranial mediastinal mass was confirmed, and a nodular interstitial pattern was identified in the lungs. Because of progressive clinical deterioration of the patient, the owners elected humane euthanasia and necropsy was performed. The mediastinal mass was consistent with an ectopic thyroid carcinoma, and the pulmonary nodules represented equine multinodular pulmonary fibrosis (EMPF). This case report describes a type of mediastinal tumor not previously described in horses. Moreover, it shows the need of including EMPF as a possible differential diagnosis for a nodular interstitial pulmonary pattern in conjunction with a mediastinal or other masses.