Thiol-disulfide oxidoreductase involved in the redox regulation of a number of cytosolic enzymes. Activates the cytosolic malate dehydrogenase (MDH) probably by reducing an interchain disulfide bond of the inactive MDH homodimer. Possesses insulin disulfide bonds reducing activity. Interacts with FBA6 (PubMed:21782461). Interacts with MDH1 (PubMed:29194485). Belongs to the thioredoxin family. Plant H-type subfamily.