Transcription factor that binds and activates the promoter of thyroid specific genes such as thyroglobulin, thyroperoxidase, and thyrotropin receptor. Crucial in the maintenance of the thyroid differentiation phenotype. May play a role in lung development and surfactant homeostasis. Forms a regulatory loop with GRHL2 that coordinates lung epithelial cell morphogenesis and differentiation (PubMed:22955271). Activates the transcription of GNRHR and plays a role in enhancing the circadian oscillation of its gene expression. Represses the transcription of the circadian transcriptional repressor NR1D1 (PubMed:22356123). Interacts with WWTR1. Thyroid, lung and brain. Expressed in lung at least from to 9.5 dpc to adulthood. Phosphorylated on serine residues by STK3/MST2. Belongs to the NK-2 homeobox family.