This study aimed to evaluate the effects of body condition scores (BCSs) on oxidative stress and thiol/disulphide homeostasis (TDH) in Holstein pregnant heifers during the transition period. A total of 36 healthy primiparous heifers in the 7th month of pregnancy, all approaching their first calving, were included in the study. The animals were allocated into three equal groups based on their BCS. The BCS measurements were performed 21 days pre-calving and on the 21st day post-calving. Pre-calving and post-calving serum levels of total thiol (TTL), native thiol (NTL) and disulphide (DSF) were quantified using standard techniques. The study revealed significant differences in BCS, TTL and DSF values among groups both pre- and post-calving (p < 0.05), whereas the changes in NTL values were statistically insignificant (p > 0.05). Overall, a decrease in BCS and NTL levels was observed, accompanied by an increase in TTL and DSF levels. Correlation analysis within groups indicated low-level relationships between changes in BCS and TTL, NTL and DSF levels. Consequently, regression analysis did not yield any statistically significant predictive models. The results showed a differential response between the loss of BCS and the oxidative stress during the periparturient period. The increased DSF levels observed during late pregnancy and early lactation indicate a deficiency in antioxidant substances in the animals. Therefore, supplementing the ration with a premix containing antioxidant substances during the transition period may provide significant benefits in terms of maintaining the BCS balance, animal welfare and herd health.