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 (By similarity). [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. Detected in cytoplasm and nucleus in late spermatids. 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 (By similarity). 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.