Antioxidant defense is considered in the context of a universal system, which, together with the components of the immune system, provides hemodynamic balance and antigenic homeostasis of the body, that is, the work of this system, represented by two practically independent links. The first line of the antioxidant system, the work of which is associated with the activity of superoxide dismutase, catalase, ceruloplasmin and a number of other enzymatic and non-enzymatic components, is considered an initial barrier, provides support at a constant controlled level of free radical oxidation. However, there is insufficient data in previous studies regarding the antioxidant defense system in patients with gastrointestinal disorders associated with thyroid disease. The purpose of the work was to establish the state of the antioxidant system in terms of total antioxidant activity and 8-isoprostane in young people with gastroesophageal reflux disease and autoimmune thyroiditis. Material and methods. The work involved 120 people with a combined course of gastroesophageal reflux disease and autoimmune thyroiditis at the age from 18 to 25 years. All patients had the same social status (students), which made it possible to equalize both physical and emotional stress, peak and stressful situations. Results and discussion. We examined the indicators of total antioxidant activity: the aggregate value of enzymes of the first level of the antioxidant system and 8-isoprostane, the level of the non-enzymatic link in antioxidant protection. These indicators depended on the morphological form of damage to the mucous membrane of the esophagus: they were more expressive in the erosive form of esophagitis. Thus, the combined course of gastroesophageal reflux disease and autoimmune thyroiditis is accompanied by changes in the indicators of the antioxidant system as a result of an active inflammatory process in the esophageal mucosa. There is a decrease in the total antioxidant activity, the first line of the antioxidant system, the value of which depends on morphological changes in the esophageal mucosa. Conclusion. An active inflammatory process in patients with gastroesophageal reflux disease and autoimmune thyroiditis leads to an increase in 8-isoprostane, an indicator of a non-enzymatic link in the antioxidant defense system