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 (By similarity). [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Homodimer. Expressed in liver, heart, testis and kidney. 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. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus. Extended N-terminus.