Naturally occurring parafollicular cell carcinoma of the thyroid in dogs. A histological and ultrastructural study.

Seven cases of parafollicular cell carcinoma in dogs were seen. The diagnosis was based on histological and ultrastructural similarities between neoplastic and normal parafollicular cells. Amyloid was not detected in any of the neoplasms by light or electron microscopy. Argyrophilic secretory granules are important in making a diagnosis of canine thyroid parafollicular cell carcinoma, even when there is no amyloid.