Beyond Type 1 Diabetes: Autoimmune Gastritis-Driven Multisystem Disease in Autoimmune Polyglandular Syndrome Type 3.

Type 1 diabetes mellitus (T1DM) is frequently managed as an isolated endocrine disorder despite being a sentinel diagnosis for broader autoimmune clustering. Autoimmune polyglandular syndrome type 3 (APS-3), defined by the coexistence of T1DM with autoimmune thyroid disease and/or autoimmune gastritis in the absence of adrenal insufficiency, remains underrecognized in routine clinical practice, particularly when manifestations extend beyond classical endocrine organs. We report a diagnostically delayed case of APS-3 in a middle-aged man whose long-standing gastrointestinal symptoms, nutritional deficiencies, and pancreatic dysfunction were repeatedly attributed to complications of diabetes rather than a unifying autoimmune process. The patient was ultimately found to have autoimmune atrophic gastritis with pernicious anemia, extreme hypergastrinemia, autoimmune hypothyroidism, and severe exocrine pancreatic insufficiency with radiologic pancreatic atrophy. Subsequent episodic biliary obstruction further complicated the diagnostic course but ultimately reinforced the need for an integrative approach. This case highlights critical gaps in endocrine-centric diagnostic frameworks, illustrates how autoimmune gastritis and pancreatic involvement can dominate the clinical picture, and underscores the importance of symptom-driven expansion of autoimmune surveillance in patients with T1DM to prevent prolonged morbidity.