Theodore was born in Constantinople in 759. He was the oldest son of Photeinos, an important financial official in the palace bureaucracy, and Theoktiste, herself the offspring of a distinguished Constantinopolitan family. The brother of Theoktiste, Theodore's uncle Platon, was an important official in the imperial financial administration. The family therefore controlled a significant portion, if not all, of the imperial financial administration during the reign of Constantine V (r. 741–775). Theodore had two younger brothers (Joseph, later Archbishop of Thessalonica, and Euthymios) and one sister, whose name we do not know.