TEF (Thyrotroph Embryonic Factor) is a member of the PAR bZIP transcription factor family. Initially identified as being specifically expressed in pituitary thyrotroph cells during embryonic development, it regulates the transcription of the thyroid-stimulating hormone (TSH-β) gene. TEF activates target genes by binding to specific DNA sequences and is involved in cell cycle regulation, proliferation, and differentiation. Recent studies have revealed that TEF is significantly downregulated in various cancers, including bladder cancer, where it functions as a tumor suppressor by inhibiting tumor cell proliferation and metastasis through the AKT/FOXOs and Hippo signaling pathways, making it a potential tumor biomarker. In addition, as a regulator of circadian rhythms, polymorphisms in the TEF gene are associated with neuropsychiatric disorders such as Parkinson's disease, depression, and sleep disorders.