Background: There is some controversy on the potential relationship between autoimmune processes and clinicopathologic features as well as prognosis of differentiated thyroid cancer (DTC), and the evidence is limited by its largely retrospective nature. We examined the relationship between the presence of autoimmune thyroiditis (AT) and 1-year thyroid cancer treatment outcomes in a large multicenter study using prospectively collected data. Methods: We included data from consecutive DTC patients enrolled in the Italian Thyroid Cancer Observatory (ITCO) database (NCT04031339). We divided the groups according to the presence (AT) or absence (no autoimmune thyroiditis [noAT]) of associated AT. We used propensity score matching to compare the clinical features and outcomes between the two groups at 1-year follow-up. Results: We included data from 4233 DTC patients, including 3172 (75%) females. The American Thyroid Association (ATA) risk levels were as follows: 51% (2160/4233) low risk, 41.3% (1750/4233) intermediate risk, and 7.6% (323/4233) high risk. There were 1552 patients (36.7%) who had AT. Before propensity score matching, AT patients were significantly younger and had a smaller and bilateral tumor (p < 0.0001). Patients with AT more frequently fell into the low- and intermediate-risk categories, while the ATA high risk was more frequent among noAT patients (p = 0.004). After propensity score matching, patients with AT more frequently showed evidence of disease (structural/biochemical incomplete response) versus excellent/indeterminate response, compared with patients without AT (7.3% vs. 4.5%, p = 0.001), with an odds ratio of 1.86 ([confidence interval: 1.3-2.6], p = 0.0001). However, when considering only structural persistence as the outcome, no statistically significant differences were observed between patients with or without AT (3.4% vs. 2.7%, p = 0.35). The elevated risk associated with the ATA intermediate and high risk at diagnosis remained consistently statistically significant. Conclusions: In this large prospective series, biochemical persistence was more frequent, at 1-year follow-up, in AT patients. However, there was no significant association between the presence of AT and structural persistence of disease. These findings may be explained by the presence of a residual thyroid tissue.

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