The protein encoded by this gene belongs to the pyridine nucleotide-disulfide oxidoreductase family; and is a member of the thioredoxin (Trx) system. Three thioredoxin reductase (TrxR) isozymes are found in mammals. TrxRs are selenocysteine-containing flavoenzymes; which reduce thioredoxins; as well as other substrates; and play a key role in redox homoeostasis. This gene encodes a mitochondrial form important for scavenging reactive oxygen species in mitochondria. It functions as a homodimer containing FAD; and selenocysteine (Sec) at the active site. Sec is encoded by UGA codon that normally signals translation termination. The 3’ UTRs of selenoprotein mRNAs contain a conserved stem-loop structure; the Sec insertion sequence (SECIS) element; which is necessary for the recognition of UGA as a Sec codon rather than as a stop signal. Alternatively spliced transcript variants encoding different isoforms; including a few localized in the cytosol and some lacking the C-terminal Sec residue; have been found for