Objective: Thyroid hormone suppression treatment (THST) is an essential modality in treating differentiated thyroid cancer (DTC). This study aimed to evaluate thiol/disulfide homeostasis with a new method in patients on THST, which causes a state of subclinical hyperthyroidism. Material and Methods: Serum thyrotrophin (TSH), free triiodothyronine (fT3), free thyroxine (fT4), duration of disease, levothyroxine dose, and radioactive iodine (RAI) dose were evaluated along with native and total thiol and disulfide levels. Results: Data of 50 patients with DTC and 41 healthy subjects were analyzed. Though native thiol and total thiol were lower in patients with DTC, the difference was not statistically significant. Disulfide was found to be 18.25 μmol/L in DTC patients and 15.23 μmol/L in the control group. The ratios of native thiol to total thiol (N/T), disulfide to native thiol (D/N), and disulfide to total thiol (D/T) were similar in the 2 groups. Disulfide, D/N, and D/T were significantly higher, and N/T was lower in patients with overt thyrotoxicosis than patients with subclinical thyrotoxicosis and the control group. Disulfide, D/N, and D/T were both positively, and N/T was negatively correlated with fT4/fT3. Conclusion: Although the thiol/disulfide balance was maintained in patients with subclinical thyrotoxicosis, there was a shift of redox status toward disulfide in patients with overt thyrotoxicosis on THST. This suggests that the potency of oxidative stress is associated with the degree of thyrotoxicosis. Considering the potentially harmful effects of oxidative stress, overt thyrotoxicosis must be avoided in patients on THST.