Purpose: This study aimed to investigate oxidative stress and histopathological changes in healthy rats after oral administration of I-131, to elucidate the mechanisms of gastrointestinal mucosal injury associated with radioiodine exposure, and provide translational insights relevant to clinical radioiodine therapy. Materials and methods: Twenty-seven rats received I-131 (9.62 × 106 Bq/100 µL) orally. Observations were made up to 7 days post-administration. Oxidative stress levels in the thyroid, small intestine, and stomach were measured using an enzyme-linked immunosorbent assay, while histopathological changes were analyzed using hematoxylin-eosin staining. Data were analyzed using one-way analysis of variance (SPSS v25). Results and conclusion: Biochemical assays showed mild and inconsistent variations in antioxidant enzyme activity. H2O2 levels remained stable, whereas thyroid SOD activity exhibited a transient ∼22-fold increase within the first hour before returning to baseline by day 7, with no corresponding elevation in CAT or GPx. Histopathological evaluation revealed marked mucosal injury in the small intestine, characterized by epithelial erosion, edema, and inflammatory infiltration within 30 min, peaking at 2–3 h, and partially reappearing on days 5–7. In contrast, gastric lesions were milder and resolved completely by day 7. The discrepancy between biochemical stability and pronounced tissue injury suggests that localized oxidative stress may occur despite unchanged bulk reactive oxygen species levels, likely due to rapid compensatory antioxidant responses in healthy tissues. Overall, these findings underscore the importance of integrating biochemical and histological endpoints for a comprehensive assessment of radiation-induced toxicity. This integrated approach also supports further investigation into lipid peroxidation, protein oxidation, glutathione redox status, and mitochondrial function to elucidate the mechanisms underlying radioiodine-induced oxidative stress. © Copyright © 2026 Taylor & Francis Group LLC.