Participates in various redox reactions through the reversible oxidation of the active center dithiol to a disulfide. The H form is known to activate a number of cytosolic enzymes. This thioredoxin cannot be used as a substrate for E.coli NADPH: thioredoxin reductase, but is a substrate of spinach ferredoxin-thioredoxin reductase and can activate NADP-MDH. Belongs to the thioredoxin family. Plant H-type subfamily.