Thyroid Adenoma and Ectopic Thyroid Carcinoma in a Guinea Pig (Cavia porcellus)

A 56-mo-old male guinea pig (Cavia porcellus) presented with gradual weight loss from 757 to 691 g during the preceding month. Cardiac-specific diagnostics detected decreased cardiac contractility, moderate cardiomegaly, pericardial effusion, and pulmonary edema. At 1 mo after presentation, the guinea pig died while under treatment. Necropsy revealed a 5 × 3 × 3-mm, tan, smooth nodule on the base of the heart. On histology, the nodule contained moderately cellular neoplastic tissue comprising cuboidal cells arranged in variably sized follicles. Immunohistochemically, the neoplastic cells were positive for thyroglobulin and thyroid transcription factor 1, consistent with ectopic thyroid carcinoma. Other significant histologic lesions included thyroid adenoma of the right thyroid gland, myocardial hypertrophy and degeneration, suppurative bronchopneumonia, and centrilobular hepatocellular degeneration. Although serum total thyroxine concentration was not evaluated, we considered that the tumors in this case were functional because concurrent lesions related to hyperthyroidism were present. In guinea pigs, reports of endocrine tumors, including thyroid tumors, have been increasing. To our knowledge, the current report represents the first documentation of ectopic thyroid carcinoma in this species.