Mixed medullary and follicular carcinoma of the thyroid.

We report a case of medullary carcinoma of the thyroid which on light microscopy showed not only the well known arrangement of cells in sheets and nests but also unequivocal follicular structures. These follicular structures are present both in the primary tumor and in lymph node metastases. Immunohistochemical investigations revealed that the cells lining the follicles produce thyroglobulin, whereas the remaining tumor tissue is positive for calcitonin and carcinoembrionic antigen. This case represents a medullary carcinoma of the thyroid with an atypical pattern consisting of both thyroglobulin and calcitonin producing cells.