Aims <p>Type 1 diabetes mellitus (T1DM) is frequently associated with other autoimmune disorders, particularly autoimmune thyroid disease (AITD) and celiac disease. Guidelines recommend screening for thyroid and celiac disease in T1DM patients. Nevertheless, in case of their positivity, indicating the presence of an autoimmune polyendocrine syndrome (APS), there is a lack of guidance on how to proceed. Should we screen these patients for other autoimmune diseases? This study’s aim was to evaluate the prevalence of additional autoimmune conditions in a cohort of T1DM patients with AITD and/or celiac disease.</p> Methods <p>This cross-sectional study includes adult T1DM patients identified through electronic medical records. Patients with concomitant AITD or celiac disease were invited to undergo an extended screening workup for autoimmune disorders.</p> Results <p>639 T1DM patients were identified, and 198 (31%) met the clinical criteria for APS (T1DM plus AITD and/or celiac disease), but only 17 (8.6%) had a formal APS diagnosis in their records. After the extended screening in 139/198 patients, 39% exhibited previously unknown autoantibody positivity, and 11.5% had new autoimmune diseases. Overall, among the patients who underwent the extended screening, 32% (44/139) were diagnosed with at least three autoimmune diseases.</p> Conclusions <p>Our study shows that APS is often under-recognized and rarely formally documented in patients with T1DM. In addition, 32% of T1DM patients with AITD or celiac disease have at least a third autoimmune disease. These findings highlight that the identification of AITD or celiac disease in T1DM marks only one stage of the diagnostic journey. APS recognition should lead to a formal diagnosis and ensure heightened clinical vigilance with tailored evaluation of further autoimmune comorbidities during lifelong patient follow-up.</p>