Thyrotroph embryonic factor (TEF) is a nuclear transcription factor which is a part of the bZIP (basic region/leucine zipper) family and PAR subfamily. TEF binds DNA as either a homodimer or heterodimer, and is known to transactivate the TSH beta promoter. TEF accumulates according to a robust circadian rhythm and is also inhibits cell growth by down-regulating beta chain expression of cytokine receptors.