TTF1 and medullary thyroid gland carcinoma: Immunohistochemistry is crucial in this context, as the tumour should be positive for S-100 protein and negative for calcitonin, carcinoembryonic antigen (CEA), thyroglobulin, thyroid transcription factor 1 (TTF1), melan-A, and melanoma-associated antigen (HMB45), which helps to rule out schwannomas, medullary thyroid cancers, and well-differentiated thyroid cancers [25].