Involved in the control of reactive oxygen species levels and the regulation of mitochondrial redox homeostasis (By similarity). Maintains thioredoxin in a reduced state. May play a role in redox-regulated cell signaling. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Inhibited by 1-chloro-2,4-dinitrobenzene and by zinc, calcium, magnesium and Fe(2+) ions. kcat is 1200 sec(-1) with thioredoxin as substrate. Optimum pH is 7.5. Homodimer. The active site is a redox-active disulfide bond. The selenocysteine residue is also essential for catalytic activity (By similarity). Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.