Oxidative stress occurs in patients with rheumatoid arthritis (RA) and plays an important role in both inflammation and destruction of RA joints. The study measured plasma levels of thioredoxin (TRX), which is induced in several cells against oxidative stress and secreted extracellularly, to validate it as a marker of oxidative stress in patients with rheumatoid arthritis (RA). They found higher levels of THR in patients with RA vs normal subjects (86.8 +/-54.1 ng/ml vs 38.6 +/-18.5 ng/ml, P<0.0001). These levels were also correlated with C-reactive protein (CRP) values. They also measured TRX in synovial fluid (SF), and it was also significantly higher in patients with RA and was also correlated with the amount of leukocytes in the SF and de serum CRP levels. TRX was also found on the surface of synovial lining layer and in leukocytes. Moreover, they also found that urinary excretion of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a biomarker of oxidative DNA damage by endogen oxygen radicals, was significantly higher in RA patients (11.55 +/- 4.71 versus 7.76 +/- 2.26 ng/mg creatinine, P<0.0001) and that TRX plasma levels were also correlated with urinary excretion of 8-OHdG. Therefore, they conclude that plasma TRX levels are a new biomarker of oxidative stress in patients with RA.