Catalyzes the transfer of electrons from NADPH to thioredoxins TRX1, TRX2 and TRX3, which in turn act as reductants of disulfide containing proteins (PubMed:8892299). Able to reduce nitroglutathione (GSNO), a compound involved in the transport of nitric oxide (NO); however, TRX1 is more efficient in reducing GSNO. Has no catalytic activity towards oxidized glutathione (GSSG) (By similarity). [thioredoxin]-dithiol + NADP(+) = [thioredoxin]-disulfide + H(+) + NADPH Binds 1 FAD per subunit. Homodimer. There are 2 isoforms resulting from alternative translation initiation. The displayed sequence is likely to represent the shorter isoform. In Plasmodium, the C-terminal redox center, which acts as the active site, is formed by two cysteines while in mammalian TRXR this redox center is composed of a cysteine-selenocysteine active site. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family. Was originally thought to be a glutathione reductase.