Which marker is NOT in the IHC panel for primary papillary thyroid carcinoma (second question)?

• A. Thyroid Transcription Factor-1
• B. Cytokeratin 19
• C. HBME-1
• D. Calcitonin

Answer: (D)

The panel for papillary thyroid carcinoma is built to confirm thyroid origin and the papillary carcinoma pattern, while helping distinguish it from other thyroid neoplasms. TTF-1 is a nuclear transcription factor expressed in thyroid follicular cells and is commonly positive in papillary thyroid carcinoma, supporting a thyroid origin. CK19 is a cytokeratin often overexpressed in papillary thyroid carcinoma and helps highlight the tumor’s epithelial phenotype. HBME-1 is frequently positive in malignant thyroid tumors, including papillary carcinoma, and can aid in separating it from benign nodules. Calcitonin, on the other hand, is produced by C cells and marks medullary thyroid carcinoma. It is not part of the typical immunohistochemical panel for primary papillary thyroid carcinoma because a positive calcitonin would suggest medullary differentiation rather than papillary tissue.