Aims: In this study, we investigated the expression of thyroid transcription factor-1 (TTF-1) in lung adenocarcinoma patients' samples and analyzed the association of TTF-1 with clinicopathological parameters, prognosis, and treatment options in patients with lung adenocarcinoma. Subjects and Methods: This retrospective study enrolled 200 patients who were histologically confirmed lung adenocarcinoma with Stage I-IV disease, between 2008 and 2015 years. The cytological archive of these hospitals' Pathology Department was searched. The available slides and the clinical information were reviewed and correlated. All analyses were conducted by SPSS version 15.0 statistical software. Results: Sixty-five (32.5%) of the patients showed TTF-1 negativity and 135 (67.5%) of them showed TTF-1 positivity. The median survival for TTF-1 positive and negative patients was 19.6 and 12.2 months, respectively. We did not find any statistical significance in-between the parameters in terms of the survival data. In TTF-1-negative group, the survival time of epidermal growth factor receptor mutation positive (P = 0.049), cytokeratin 7 (CK7) positive (P = 0.009) patients and those who had received curative radiotherapy (P = 0.028) was significantly better as compared to TTF-1-positive group. We also analyzed the relation between TTF-1 and survival outcome or chemotherapy selection in Stage IV disease. We could not identify any correlation between TTF-1 and survival outcome or treatment selection. Conclusions: This study suggests that TTF-1 is not a favorable prognostic factor in lung adenocarcinoma patients. The prognostic role of CK7 and relationship between TFF-1 expression in lung adenocarcinoma and predictive role of TTF-1 expression for the selection of first-line treatment in Stage IV lung adenocarcinoma should be validated in prospective and randomized studies.