Background. Most experts agree that duodenal ulcers are much more common in children. In recent years, the research interest in the relationship between the thyroid system and duodenal ulcer processes has become evident. The purpose of the study is to investigate the thyroid functional state in children with duodenal ulcer. Materials and methods. The observation group consisted of 56 children aged 7 to 18 years with a verified duodenal ulcer and 15 healthy children. All pediatric patients underwent fibroesophagogastroduodenoscopy with a targeted thyroid biopsy and examination for the presence of the Helicobacter pylori bacterium, intragastric pH-metry, ultrasound examination of the abdominal cavity and thyroid gland; the concentration of thyroid hormones (thyroid-stimulating hormone, T3, T4) was also studied. Reliability was assessed using Student’s t-test. The difference was considered significant at p<0.05. Results. The average age of children with duodenal ulcer was 13.1±3.1 years. In all age categories, duodenal ulcer occurred more often in boys— 58.9%. During an in-depth ultrasound examination, the structure of the thyroid gland in 52 (92.8%) children was homogeneous, in 4 (7.1%) individuals it was heterogeneous, but without nodular formations. Serum concentrations of thyroid-stimulating hormone and T3 in children with duodenal ulcer also did not differ from the average levels in children of the comparison group. At the same time, a decrease in the concentration of T4 was noted, which can be considered as a variant of euthyroid sick syndrome— the syndrome of reduced T4. The average serum level of T4 (32.67±3.84 nmol/l) was significantly lower (p=0.001) in children infected with Helicobacter pylori than in those without infection (39.45±4.23 nmol/l) and healthy individuals (89.12±5.91 nmol/l). Conclusions. Children with a duodenal ulcer have a violation of the thyroid status due to a decrease in the thyroxine concentration. A functional shift in the thyroid status in these patients can be considered as a prognostically unfavorable sign of possible deterioration of the ulcer course.