MON-566 Prevalence and Causes of Non-Type 1 Diabetes Mellitus in Adults with Normal Body Mass Index

Abstract Disclosure: N. Chahal: None. S.C. Russo: None. A. Clifford: None. C.A. Dash: None. J.E. Johnson: None. A.K. Arpante: None. A.K. Gattu: None. S.J. Cromer: None. M.S. Udler: None. L.T. Fourman: None. Background: Diabetes mellitus is typically categorized as type 1, resulting from autoimmune destruction of the pancreas, and type 2, driven by insulin resistance often in the setting of excess adiposity. Non-type 1 diabetes in adults with normal BMI represents a clinical presentation that does not align with this classification. Current guidelines lack specific recommendations for evaluating such cases, which may lead to failure to detect underlying etiologies. This study aimed to characterize the prevalence, differential diagnosis, and real-world workup of non-type 1 diabetes in adults with normal BMI. Methods: Leveraging a patient data registry at a tertiary academic center (2013-2023), we identified 34,180 adults aged 18-65 years with HbA1c ≥7% without a diagnosis of type 1 diabetes or positive GAD65 antibodies. Restricting our sample to those with HbA1c ≥7% and BMI within 1 year yielded 16,379 individuals. We next excluded those with any history of overweight/obesity (BMI ≥25 kg/m2, or ≥23 kg/m2 for Asians), resulting in 569 adults whose records were manually reviewed by an endocrinologist. Exclusions were applied for lack of true hyperglycemia (n=7), type 1 diabetes (n=20), insufficient information (n=102), or elevated BMI (n=156). Among the remaining 284 adults confirmed to have HbA1c ≥7% and normal BMI, we determined the underlying cause of diabetes from all available information. Cases of diabetes without an identifiable cause were classified as undifferentiated. Results: The prevalence of normal BMI in adults with non-type 1 diabetes was 1.7% (N=284/16,250). The most common attributable causes were organ transplant (12.0%), cystic fibrosis (10.2%), pancreatitis (9.9%), and glucocorticoid use (9.5%). Undifferentiated diabetes accounted for 38.4% of cases of which only 33.9% underwent diagnostic workup (e.g., insulin, C-peptide, cortisol) and <1% received genetic testing. Compared to cause-attributed cases, individuals with undifferentiated diabetes were older at diagnosis (47.6±8.2 vs. 44.1±13.3 years, P<0.01) and had higher BMI (21.8±1.8 vs. 20.5±2.3 kg/m2, P<0.01). Black (21.1% vs. 9.1%) and Asian (23.9% vs. 9.1%) individuals were disproportionately represented in the undifferentiated group (P<0.0001). Conclusions: Non-type 1 diabetes in adults with normal BMI is a rare yet distinct entity that often lacks an identifiable cause. Diagnostic workup was performed in only a minority of undifferentiated cases, indicating a blind spot in which clinicians may not recognize these cases as atypical to trigger further evaluation. The observed racial differences between undifferentiated and cause-attributed cases may reflect distinct genetic predispositions and/or social disparities between racial groups. Further research is needed to uncover the genetic and molecular underpinnings of non-type 1 diabetes in adults with normal BMI to enable more personalized care. Presentation: Monday, July 14, 2025