Abstract Disclosure: L.M. Pereira: None. C. Neves: None. J. Neves: None. L. Delgado: None. D. Carvalho: None. Introduction: Autoimmune thyroiditis (AIT) may be associated with physical, psychological and behavioral symptoms even in patients achieving normal TSH and free T4 levels with levothyroxine treatment. Objective: Our aim was to evaluate the associations the resistance to thyroid hormone, assessed by the Parametric Thyroid Feedback Quantile-Based Index (PTFQI), and laboratorial parameters with Quality of life (QoL) in AIT. Methods: We evaluated 145 patients with AIT treated with levothyroxine followed in outpatient clinic. We assessed lipid profile, high-sensitivity C-reactive protein, B12 vitamin, folic acid, thyroid antibodies, thyroid function tests and calculated the index PTFQI of resistance to thyroid hormones. The following questionnaires were applied to participants: the Thyroid Dependent Quality of Life questionnaire (ThyDQoL), the Thyroid Symptom Rating Questionnaire (ThyTSQ), the Thyroid Treatment Satisfaction Questionnaire (ThyTSQ) and the Brief Symptom Inventory (BSI). Statistical analysis was performed with the One-way ANOVA test and Pearson’s correlation test. P values ≤ 0.05 were considered as statistically significant. Results: In this sample, the age was 54.4 ± 15.3 years, 89.7 % were female, BMI was 28.4 ± 5.2 Kg/m2, TSH was 2.69 ± 8.48 µUl/mL and FT4 was 1.15 ± 0.40 ng/dL. Patients reported a mean QoL value of -2.11 points (range from -9 to 1). In regard to the ThySRQ, 46.2 % of patients reported at least moderately memory problems, 50.4% of patients reported at least being moderately tired and 62.7 % showed some type of depressive feelings. Regarding ThyTSQ, 83.5 % demonstrated being satisfied with treatment and 82 % believe that treatment is working well. We found that PTFQI correlates positively with thyroid peroxidase antibodies (r = 0.19; p = 0.03) and negatively with triglycerides (r = -0.26; p = 0.002) and BMI (r = -0.21; p = 0.01). In relation with symptomatology, we observed negative correlations among PTFQI and constipation (r = -0.18; p = 0.02), skin problems (r = -0.27; p = 0.01), nail problems (r = -0.19; p = 0.01) and dizziness symptoms (r = -0.18; p = 0.03). Conclusions: In this study, we observed a high prevalence of memory problems, tiredness and depressive feelings in patients with AIT treated with levothyroxine. We also found that PTFQI is associated with physical and psychological symptoms. Further studies are needed to analyze the role of autoimmunity and PTFQI for the worsening of the QoL in patients with AIT. Presentation: Friday, June 16, 2023