Introduction: Insulinomas are the most common cause of hypoglycemia related to endogenous hyperinsulinism. It occurs in 1-4 people per million in the general population. Symptoms include diaphoresis, palpitations, tremors and even confusion or behavioral/personality changes.1 The small size of insulinomas presents a challenge in diagnosis via standard imaging techniques.2 Case Description/Methods: A 47-year-old healthcare worker with a history of goiter presented with 6 months of memory problems and associated lightheadedness, tremors, and blurry vision. She endorsed a 15-pound weight gain in the past month. She also recalled lapses in memory such as forgetting where she parked her car at the grocery store and more dangerously occurring during work when she was not able to remember if she administered medications to patients appropriately. Historical episodes of hypoglycemia had been ongoing for the past 2 years. Baseline labs included a mildy elevated c-peptide, normal IGF-2, TSH, and cortisol, and negative insulin antibody and sulfonyurea screen. Physical examination was normal. MRI showed no pancreatic abnormalities and she was subsequently admitted to our hospital for 72hour inpatient fast. The fast ended at 22 hours due symptomatic hypoglycemia that resolved after administration of glucagon. Labs revealed blood glucose of 42, insulin 9 mU/L, beta-hydroxybutyrate 0.4 mg/dL, C-peptide 1.4 ng/L , proinsulin 40.5 pmol/L which were all diagnostic for insulinoma. CT abdomen showed no enhancing pancreatic masses or metastatic lesions. Thus, the patient underwent endoscopic ultrasound (EUS) fine-needle aspiration which identified an 8x5 mm lesion in the tail of the pancreas. Pathology showed a well-differentiated neuroendocrine tumor, and the patient underwent radiofrequency ablation (RFA). Four months later, the patient was no longer symptomatic and no biochemical evidence of hyperinsulinemia remained on laboratory evaluation. Discussion: Insulinomas remain a diagnostic challenge, due to their rare presentation, nonspecific symptoms and small size. Although most insulinomas are benign with survival rate of 95% they require surgical intervention or radiofrequency ablation to improve survival. Diagnosis of a suspected insulinoma can be made via labs during a supervised, prolonged fast and accurate localization of smaller tumors may require minimally invasive procedures, like EUS. 2