During the past decade, case reports have indicated the existence of mixed follicular and medullary carcinomas: in one instance, positive immunostaining occurred with both antithyroglobulin and anticalcitonin in the same tumour cells. The identification of these mixed tumours has suggested that some follicular thyroid cells may have the capacity to differentiate into a C-cell phenotype or, alternatively, that some cells of ultimobranchial origin may differentiate to express a follicular phenotype.
