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 (By similarity). 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 CNS. Expressed in restricted regions of the developing brain within the diencephalon, in parts of the hypothalamus and neurohypophysis, and in the telencephalon. Expression oscillates in a diurnal and melatonin-dependent fashion in the preoptic area (POA) region in the hypothalamus, with maximal expression attained during the dark phase of the light/dark cycle. Phosphorylated on serine residues by STK3/MST2. Belongs to the NK-2 homeobox family.