SAT148 A Rare Case Of Proinsulinoma In A Patient With Type 2 Diabetes Mellitus

Disclosure: E. Tirthani: None. F. Erenler: None. Background: A proinsulinoma is a very rare neuroendocrine tumor of the pancreas, which predominantly secretes proinsulin, the precursor hormone of insulin. Even though not as potent as insulin, proinsulin in higher amounts can bind to insulin receptors and result in hypoglycemia. Most patients present with initial hypoglycemia symptoms at the time of diagnosis of the tumor. The diagnosis becomes challenging in patients with underlying type 2 diabetes mellitus (T2DM) who may initially have a spontaneous remission of diabetes, followed by hypoglycemia episodes later in the disease course. Clinical Case: A 66-year-old man presented to our clinic in October 2020 with frequent episodes of dizziness and diaphoresis, which were relieved by eating food. The patient's symptoms began in 2018, occurred during the daytime, and were exacerbated by exercise. He had a past medical history of T2DM diagnosed in 2009 (HbA1c=10%) and treated with glipizide and metformin; however, over the years, his HbA1c improved, and glipizide was discontinued. By 2018 his HbA1c was down to 5.9%, and metformin was also stopped. The patient denied any change in his weight, diet, or exercise frequency during this time course. A fingerstick blood glucose (BG) checked during one of these episodes was 51 mg/dL. An abdominal CT scan in August 2020 revealed a 12x13 mm enhancing mass within the neck of the pancreas concerning for a neuroendocrine tumor. The patient was further evaluated with a 14-day continuous glucose monitor (CGM), which showed that the patient's BG was <70 mg/dL 31% of the time and <55 mg/dL 6% of the time. Additional fasting blood work showed a proinsulin level of 191.3 pmol/L (normal ≤18 pmol/L), insulin 15.5 uIU/mL (2-25 uIU/mL), BG 103mg/dL, C-peptide 2.44 ng/mL (0.80-3.85 ng/mL) and chromogranin A 124 ng/mL (25-140 ng/mL) which suggested the diagnosis of proinsulinoma. A Gallium-DOTATATE scan showed a localized tumor within the pancreas with no metastasis. The patient underwent enucleation of the tumor in November 2020. Pathology showed a well-differentiated neuroendocrine tumor staged T1 pN0 with immunohistochemistry staining diffusely positive for chromogranin, synaptophysin, and insulin with a Ki67 index <3%. In January 2021, at a post-surgery follow-up visit, blood work showed HbA1c 7.1%, proinsulin 31 pmol/L, insulin 13.4 uIU/mL, C-peptide 1.6 ng/mL, BG 193 mg/dl, and chromogranin A 118 ng/mL. HbA1c continued to trend up to 8.0% in January 2022, confirming the resurgence of diabetes, and metformin and pioglitazone were started. Conclusion: Per our literature review, this is the first reported case of a proinsulinoma in a patient with T2DM. In a patient with uncontrolled T2DM, an unexpected amelioration of the disease should raise concern for insulin-secreting tumors, and further workup should be considered. A CGM can be a valuable tool to assess the severity of hypoglycemia in patients with these tumors. Presentation: Saturday, June 17, 2023