SUN-289 A Case of Insulinoma After an Equivocal 72-Hour Fasting Test

Abstract Disclosure: P. Pile: None. S. Shteyman: None. F.S. Celi: None. Introduction: I nsulinoma is a rare neuroendocrine tumor which secretes insulin constitutively, leading to hypoglycemia. A 72-hour fasting test is the diagnostic gold standard and surgery is the first-line treatment. We present a case of a patient with symptomatic hypoglycemia with an equivocal 72-hour fast with histological confirmation of insulinoma. Case Presentation An 82-year-old female with a history of hypothyroidism was referred to endocrinology for symptomatic hypoglycemia in the absence of history of diabetes, bariatric surgery, obesity or weight gain. She reported fatigue and had a fasting serum glucose of 36 mg/dL (70-99 mg/dL). Repeat fasting serum glucose was 33 mg/dL and outpatient hypoglycemia work-up showed AM cortisol 12 ug/dL (7-23 ug/dL), ACTH 9.8 pg/mL (7.2-63.3 pg/mL), proinsulin <2 pmol/L (<7.2 pmol/L), C-peptide 2.1 ng/mL (0.5-3.3 ng/mL), insulin 8 (3-25 uIU/mL), negative sulfonylurea screen. A continuous glucose monitor (CGM) recorded confirmed symptomatic hypoglycemic episodes with readings of 50-60 mg/dL. A 72-hour fast test was completed after 20 hours with a glucose nadir of 42 mg/dL, insulin levels 4-5 uIU/mL, C-peptide level 1.4-1.6 ng/mL, proinsulin <2 pmol/L, beta hydroxybutyrate (BHB) level 0.5 mmol/L (0.0-0.3 mmol/L). IGF-2 level was 344 ng/mL (180-580 ng/mL). MRI pancreas protocol revealed an 8 mm focus of increased enhancement in the late arterial and portal venous phases in the pancreatic tail which corresponds to the increased activity seen in the PET/CT Ga-68 dotatate scan that was done. Endoscopic ultrasound needle aspiration was suggestive of a neuroendocrine tumor. She underwent a distal pancreatectomy. Histology revealed a well-differentiated neuroendocrine tumor, consistent with insulinoma. Discussion: This case is unique with an equivocal 72-hour fast in a patient with histologically confirmed insulinoma. Potentially, normal beta cell function in well differentiated tumor cells may be able to maintain partial hypoglycemic suppression. Per clinical practice guidelines, during a 72-hour fast, symptoms and signs of hypoglycemia with an insulin of at least 3.0 uIU/mL and proinsulin of at least 5 pmol/L document endogenous hyperinsulinism. The insulin cutoff was satisfied, but proinsulin was below the cutoff. In addition, BHB is typically decreased in insulinoma due to the excessive insulin produced by the tumor suppressing ketogenesis. In this case, BHB was mildly elevated which could be attributed to increased insulin resistance due to prolonged exposure of the cells to high insulin levels. Conclusion: This case highlights the importance of interpreting 72-hour fast results in patients with high index of suspicion for insulinoma. Radiologic imaging also plays a crucial role in its diagnosis. Larger studies can also be done regarding evaluation of diagnostic performance of cutoffs of serum insulin, proinsulin and BHB in a 72-hour fast. Presentation: Sunday, July 13, 2025