Reduces disulfide protein thioredoxin (Trx) to its dithiol- containing form (PubMed:8577704). Homodimeric flavoprotein involved in the regulation of cellular redox reactions, growth and differentiation. A selenocysteine residue at the C-terminal active site is essential for catalysis (Probable). Also has reductase activity on hydrogen peroxide (H2O2) (PubMed:10849437). {ECO:0000269|PubMed:10849437, ECO:0000269|PubMed:8577704, ECO:0000305|PubMed:17512005}. [Isoform 1]: Induces actin and tubulin polymerization, leading to formation of cell membrane protrusions. . [Isoform 4]: Enhances the transcriptional activity of estrogen receptors ESR1 and ESR2. . [Isoform 5]: Enhances the transcriptional activity of the estrogen receptor ESR2 only (PubMed:15199063). Mediates cell death induced by a combination of interferon-beta and retinoic acid (PubMed:9774665). {ECO:0000269|PubMed:15199063, ECO:0000269|PubMed:9774665}. This is the function of TXNRD1 (thioredoxin reductase 1, Ensembl gene identifier ENSG00000198431).