Involved in the control of reactive oxygen species levels and the regulation of mitochondrial redox homeostasis (By similarity). Maintains mitochondrial thioredoxin in a reduced state. May play a role in redox-regulated cell signaling. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Homodimer. Expressed in liver, kidney, adrenal gland and heart. The measured range is 37-526. 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.