Anti-TTF-1 (Thyroid Transcription Factor 1) is beneficial for distinguishing primary lung adenocarcinoma from carcinomas that are metastatic and come from other organs rather than the thyroid1-2, germ cell tumors3, and malignant mesothelioma.4-5 It can also aid in separating small cell lung carcinoma from lymphoid infiltrates. TTF-1 staining can be found in thyroid and tumors that originate from the thyroid.7 TTF-1 immunostaining can effectively differentiate between adenocarcinomas that originate in the lungs and those that do not in cases of malignant effusions.8 TTF-1 staining is highly dependable for identifying whether a brain metastasis is from a pulmonary or nonpulmonary origin, especially for adenocarcinomas and large-cell carcinomas.