Expression of keratin 19 distinguishes papillary thyroid carcinoma from follicular carcinomas and follicular thyroid adenoma.

Keratin expression with the use of chain-specific monoclonal antikeratin antibodies was investigated in normal thyroid tissue (n = 4), colloid nodules (n = 19), follicular thyroid adenomas (n = 18), follicular carcinomas (n = 10), and papillary carcinomas (n = 12). Frozen sections were stained with monoclonal antibodies M20 (keratin 8), M9 (keratin 18), and LP2K (keratin 19) with the use of the indirect immunoperoxidase technique. The immunohistochemical findings showed that the expression of keratins 8 and 18 was equally extensive in all normal, benign, and malignant lesions tested. In contrast, different staining patterns were observed with the use of monoclonal antibody to keratin 19. Follicular carcinomas were only focally stained with this antibody or were not reactive at all. Keratin 19, however, was present in all the tumor cells of papillary tissues and in a moderate amount of cells of nonneoplastic thyroid lesions and follicular adenomas. In papillary carcinoma, an identical homogeneous expression of keratin 19 was observed in both papillary and follicular structures, which suggests a common cellular origin. These results show that immunohistochemical staining with the use of monoclonal antibody against keratin 19 is useful to distinguish papillary thyroid carcinomas from follicular adenomas and follicular thyroid carcinomas.