TTF1 (thyroid transcription factor 1) is a transcription factor that plays critical roles in both thyroid development and lung adenocarcinoma biology. In thyroid development, TTF1 mutations are associated with congenital hypothyroidism and thyroid dysgenesis, where mutations in TTF1 along with other transcription factors like PAX8 cause defects in thyroid organogenesis resulting in hypoplastic, ectopic, or absent thyroid glands 1. TTF1 mutations have also been identified in Chinese patients with congenital hypothyroidism, contributing to thyroid dysgenesis cases 2. In lung cancer, TTF1 serves as both a diagnostic marker and prognostic factor with complex roles. TTF1 protein expression is associated with favorable prognosis in lung adenocarcinoma, while TTF1 copy number alterations (amplification and polysomy) are independent indicators of unfavorable prognosis 3. TTF1-negative lung adenocarcinomas represent a distinct phenotype with predominance in male smokers, worse performance status, and greater metastatic burden 4. Additionally, TTF1 expression status influences chemotherapy response, with TTF1-negative patients showing improved outcomes with non-pemetrexed regimens compared to pemetrexed-based therapy 4. Genetic variants in TTF1 have also been associated with increased papillary thyroid cancer risk 5.