Displays thioredoxin reductase, glutaredoxin and glutathione reductase activities. Catalyzes disulfide bond isomerization. Promotes disulfide bond formation between GPX4 and various sperm proteins and may play a role in sperm maturation by promoting formation of sperm structural components. [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. Detected in cytoplasm and nucleus in late spermatids. Expressed preferentially in testis where it is found in spermatids and spermatocytes but not in sperm. In elongating spermatids, expressed at the site of mitochondrial sheath formation. Low levels in other tissues including heart, lung, liver, kidney, brain, muscle and prostate. Accumulates in the testis after puberty. Not detected in 20-day-old mice but highly expressed in testes of 7-month-old mice. The N-terminal glutaredoxin domain does not contain the C-X-X-C redox-active motif normally found in glutaredoxins but activity may be mediated through a single cysteine. The C-terminal Cys-Sec motif of one subunit of the homodimer may transfer electrons from the thiol-disulfide center to the glutaredoxin domain of the other subunit. The thioredoxin reductase 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. This sequence initiates at a CTG codon. Truncated C-terminus. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon. Truncated C-terminus. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon. Truncated C-terminus. Unusual initiator. The initiator methionine is coded by a non-canonical CTG leucine codon.