Bone Marrow Toxicity Caused by Estrogen Toxicity in a Yorkshire Terrier with Leydig Cell Tumor

A 15-year-old intact Yorkshire terrier was presented with anorexia, lethargy, and a pale mucous membrane. A physical examination one year ago revealed right testis mass and subcutaneous petechia. Blood work revealed severe thrombocytopenia and mild anemia, and no abnormalities were found in serum chemistry or ultrasonography. The preoperative serum estrogen concentration was moderately elevated. The enlarged testis was surgically removed. A well-encapsulated mass composed of polyhedral or round with abundant eosinophilic cytoplasm containing fine granular or vacuolation were found in a histological examination of the removed tissue. The nuclei of tumor cells were round, and mitotic figures were low but neoplastic cells showed a mild invasive tendency to adjacent tissues with contained neoplastic cell emboli in one lymphatic lumen. A diagnosis of a malignant Leydig cell tumor was made. The patient recovered from surgery uneventfully, but his condition worsened despite repeated transfusions and supportive therapy, and he was euthanized according to the owner's decision. Leydig cell tumor should be included in estrogen toxicity associated with testicular mass.