Involved in the control of reactive oxygen species levels and the regulation of mitochondrial redox homeostasis (PubMed:24601690). Maintains thioredoxin in a reduced state. May play a role in redox-regulated cell signaling. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Homodimer. Highly expressed in the prostate, ovary, liver, testis, uterus, colon and small intestine. Intermediate levels in brain, skeletal muscle, heart and spleen. Low levels in placenta, pancreas, thymus and peripheral blood leukocytes. According to PubMed:10608886, high levels in kidney, whereas according to PubMed:9923614, levels are low. High expression is observed in the adrenal cortex (PubMed:24601690). The disease may be caused by variants affecting the gene represented in this entry. The active site is a redox-active disulfide bond. The selenocysteine residue is essential for enzymatic activity (By similarity). Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family. Truncated N-terminus. Truncated C-terminus.