Thioredoxin (Trxs) is a group of small molecules widely present in all living cells and is a key regulator of cellular REDOX balance. The mammalian Trx family has three members: TRX-1, TRX-2, and TrxL. Trx-1 is the only thioredoxin that exists outside the cell and is secreted by lymphocytes, hepatocytes, fibroblasts, and a variety of tumor cells. The concentration of Trx-1 in plasma can reach 6 nM. It is found mainly in the cytoplasm of the cell, with small amounts in the nucleus and places associated with the outer surface of the cell. Trx-1 expression is increased under various stress conditions such as hypoxia, elevated hydrogen peroxide concentration, photochemical oxidative stress, viral and bacterial infections. Biological functions of Trx-1 include growth factor activity, antioxidant properties, reduction-equivalent cofactors, and transcriptional regulation. The synovial tissue of rheumatoid arthritis patients has increased Trx-1 levels under oxidative stress conditions. Plasma Trx-1 levels correlate with disease severity, suggesting that Trx-1 can be used as a biomarker for pathological conditions.