Abstract Disclosure: B.S. Neutel: None. G. Grani: None. S. D'elia: None. C. Durante: None. A.G. Gianoukakis: None. The prevalence and significance of newly developed (de novo) thyroglobulin antibodies (dnTgAb) in patients with differentiated thyroid cancer (DTC) who were initially thyroglobulin antibody (TgAb) negative, is not well established. We sought to examine the phenomenon of dnTgAb using the Italian Thyroid Cancer Observatory (ITCO) database. Beginning in 2013, the ITCO repository prospectively collected clinical data on DTC management in a consecutive series of newly diagnosed patients treated in large academic centers distributed across Italy. Patients were managed by treating physicians based on perceived best practice. Pre-specified parameters were collected. Clinical status, investigations, and treatments were updated periodically. We formulated inclusion and exclusion criteria to mirror previously published literature studying dnTgAb. Patients included in the analysis were initially TgAb negative at their first postoperative check and had no evidence of persistent disease at their 1 year follow up visit. The dnTgAb group included patients who were initially TgAb negative and then developed TgAb on at least one measurement from the second postoperative check or later. All patients included in the study had at least 3 years of follow up and 3 TgAb measurements. At the time of analysis, 5360 patients were without persistent disease at 1 year. 2107 of these patients had a least 3 years of follow up and 3 TgAb measurements. 334 patients with positive TgAb at their first lab check and an additional 124 patient with incomplete data were excluded. 1649 patients met criteria for inclusion. Median follow up time was 1.85 years. DnTgAb occurred in 4.3% of patients (n=71). We found no statically significant difference in gender, tumor size, tumor focality, extrathyroidal extension, AJCC stage and ATA risk level between the persistently negative TgAb group and the dnTgAb group. Only primary tumor vascular invasion occurred at a higher rate in patients within the dnTgAb group when compared to the persistently negative group (p=0.012). Structural DTC recurrences occurred in 1.4% (n=22) of persistently TgAb negative patients and 5.6% (n=4) of patients with dnTgAb (p=0.02).This is the largest and first prospective cohort used to evaluate the prevalence and clinical significance of dnTgAb. DnTgAb occurred at a clinically relevant rate and there was a statistically significant relationship between detection of dnTgAb and structural DTC recurrence. The study is limited by the small overall recurrence rates and the exclusion of patients with less than 3 years of follow up. Presentation: 6/3/2024
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