Central component in the thioredoxin system. Reduces thioredoxins 1 and 2. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. The active site is a redox-active disulfide bond. Present with 292000 molecules/cell in log phase SD medium. Belongs to the class-II pyridine nucleotide-disulfide oxidoreductase family.