Differentiated thyroid carcinoma, intermediate type: a new tumor entity with features of follicular and parafollicular cell carcinoma.

A group of differentiated thyroid carcinomas with morphologic and immunohistochemical traits of both follicular and medullary carcinoma is described. This group consists of tumors characterized by a solid and/or cribriform histologic pattern with a more or less pronounced admixture of follicular structures, immunohistochemical evidence for the production of thyroglobulin and one or more of the three neurohormonal peptides neurotensin, somatostatin, and calcitonin, and absence of amyloid. These tumors were selected from thyroid carcinomas that had been diagnosed, according to current histopathologic criteria, as differentiated follicular carcinomas. A series of pure follicular carcinomas, devoid of solid or cribriform structures, was found to contain thyroglobulin immunoreactive tumor cells only, whereas a series of classic medullary carcinoma with amyloid stroma revealed specific staining for all three neurohormonal peptides but not for thyroglobulin. The results suggest that differentiated thyroid carcinoma can be regarded as a continuous spectrum of tumor types, with pure follicular carcinomas and classic medullary carcinoma representing the two extremes and the tumors with biphasic features representing a broad intermediate group. The authors suggest that this group be designated "differentiated thyroid carcinoma, intermediate type."