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. Thyroid, lung and CNS. Phosphorylated on serine residues. Belongs to the NK-2 homeobox family.