Objective: The aim of this study was to investigate the role of dynamic thiol-disulfide homeostasis as a new oxidative stress parameter in patients with major depressive disorder (MDD).

Material and Methods: Sixty-three patients with their first episode of untreated MDD, and 61 healthy volunteers were included in the study. Serum thiol/disulfide levels were measured in fasting blood samples. The data were compared between the two groups.

Results: No significant difference was observed between the two groups in terms of age, gender distribution, or body mass index. Plasma native and total thiol levels were lower in the MDD group compared to those in the controls (p=0.004, p=0.001). No significant differences were observed between the groups in terms of disulfide, the disulfide/native thiol, the disulfide/total thiol or the native thiol/total thiol ratio (p>0.05). No relationship was detected between these parameters.

Conclusion: As far as we know, this is the first study to evaluate changes in thiol/disulfide homeostasis in male and female patients with MDD. Our data show that thiol levels decrease during the first episode of untreated depression. Thiol/disulfide homeostasis may be useful as a biomarker for depression after long-term follow-up and treatment studies.