Which marker is NOT included in the IHC panel for primary follicular thyroid carcinoma (FTC)?

• A. PAX8
• B. Thyroid Transcription Factor-1
• C. Cytokeratin 19
• D. Galectin-3

Answer: (D)

In immunohistochemistry for thyroid lesions, you want markers that reliably show thyroid origin and help differentiate malignant from benign follicular-pattern tumors. PAX8 and thyroid transcription factor-1 (TTF-1) are strong lineage markers; their positivity confirms the tumor arises from thyroid tissue. Cytokeratin 19 can aid interpretation by highlighting patterns that help distinguish papillary features from follicular lesions, providing supportive context in the assessment of follicular-pattern tumors.

Galectin-3, while associated with malignancy in thyroid tumors, has variable expression in follicular thyroid carcinoma and is not as consistently informative in this specific setting. Its reliability is limited, so it isn’t routinely included in the primary IHC panel for evaluating follicular thyroid carcinoma.