Background. Among the chronic diseases, chronic kidney failure is one of diseases that have the most difficulty in coping with oxidative stress due to the deterioration of the antioxidant system balance in the body. Beyond being a vitamin, 1 alpha,25-dihydroxycholecalciferol (vitamin D3) is a molecule that positively or negatively affects many enzymes which are in protein structures. Thioredoxin (TRX), which has an important role in the antioxidant system, is one of these proteins. By conducting this study, we wanted to emphasize the role of vitamin D3 in reducing the oxidative stress load on patients undergoing peritoneal dialysis (PD) via serum TRX level measurement. Methods. In this study, we evaluated the medical treatments of 69 PD patients who were followed up routinely. The patients were divided into 2 groups according to whether they used vitamin D3 or not. 49 of our patients were using vitamin D3. While requesting routine laboratory tests, we reserved a separate serum sample to measure serum TRX levels by double-antibody sandwich enzyme-linked immunosorbent assay for all patients. Results. Only one parameter has a significant statistical relationship with serum TRX level and the treatment protocol. The serum TRX level was significantly higher (211,62 U/l +/- 314,46) in the group receiving vitamin D3 compared to the group which is not using Vitamin D3 (101,63 U/l +/- 215,03) (p < 0,006). Conclusion. This study highlights the importance of appropriate dose of vitamin D3 replacement especially in PD patients who are under intense oxidative stress compared to healthy individuals.