Efficient electron donor for the essential enzyme ribonucleotide reductase. Is also able to reduce the interchain disulfide bridges of insulin (By similarity). [protein]-dithiol + NAD(+) = [protein]-disulfide + H(+) + NADH [protein]-dithiol + NADP(+) = [protein]-disulfide + H(+) + NADPH Belongs to the thioredoxin family.