Which marker is NOT part of the IHC panel for primary mixed thyroid tumor (second question)?

• A. Thyroid Transcription Factor-1
• B. Thyroglobulin
• C. Calcitonin
• D. Galectin-3

Answer: (D)

The main idea here is identifying which immunohistochemical markers are most informative for characterizing a primary mixed thyroid tumor. TTF-1 is a nuclear transcription factor expressed in thyroid tissue, so it helps confirm thyroid origin and supports a follicular/ductal lineage in the tumor. Thyroglobulin is produced by thyroid follicular cells, making it a key marker to establish thyroid differentiation in the tumor’s follicular components. Calcitonin marks medullary thyroid carcinoma because it is produced by the parafollicular C cells, which is crucial when a tumor has a medullary component or mixed features. Galectin-3, while useful as an adjunct marker in thyroid pathology and in some contexts helps differentiate malignant versus benign follicular-pattern lesions, is not a core, definitive indicator of thyroid origin or of specific thyroid lineages. It is not routinely included in the standard panel used to define a primary mixed thyroid tumor.