[thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Activity was very low in selenium-depleted cells, but increased 4-fold to the same level as in selenium-sufficient cells for 70 hours after the addition of 10 nm selenite. Homodimer. The N-terminus is blocked. The thioredoxin reductase active site is a redox-active disulfide bond. The selenocysteine residue is also essential for catalytic activity. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.