Title: Effect of replacement therapy and supplementation on quality of life in patients with autoimmune thyroiditis. Objectives: The objectives of this study were to examine the impact of replacement therapy and supplementation on the quality of life in patients with autoimmune thyroiditis. The study focused on several specific aspects within these objectives: determining the relationship between adherence to replacement therapy and quality of life, assessing the effectiveness of supplementation in improving quality of life, and identifying key factors that contribute to better quality of life in patients. Study design: The study was conducted as a cross-sectional study. Participants and methods: The study involved 165 patients with autoimmune thyroiditis who are under control at the Clinical Medical Center Osijek. Quality of life was assessed using the SF-36 questionnaire, along with the collection of anthropometric data and information on antibodies, supplements, and thyroid disease status. Results: The results showed that patients who adhere to replacement therapy have better control of thyroid hormone levels, which is associated with better physical health. However, no significant differences were observed in the domains of emotional functioning and mental health regarding adherence to therapy. The use of vitamin D, although expected to be beneficial, showed an opposite effect, suggesting that patients with more severe symptoms are more likely to use this supplement. Selenium showed a positive effect in reducing pain symptoms, while zinc and vitamin B12 did not have a significant impact on quality of life. Higher age and body mass index (BMI) have shown a negative impact on the quality of life of patients Conclusion: These findings highlight the importance of an individualized approach to the treatment of autoimmune thyroiditis, including adjusting the dose of replacement therapy and carefully considering the use of supplements. Further research is needed to better understand the specific effects of different therapeutic approaches and supplements.