A Curious Case of Hypoglycemia: A Rare Occurrence of Insulin Autoimmune Syndrome

Background: Hypoglycemia can be challenging, requiring close monitoring and evaluation. Although treating diabetes can cause hypoglycemia, the coexistence of autoimmune syndromes contributes to rare etiologies. They are characterized by elevated insulin levels with either insulin autoantibodies (IAA) or insulin receptor antibodies (IRA). It has been observed commonly in Japan but is scarce among non-Asian groups. We present a unique case of insulin autoimmune syndrome (IAS) that posed a diagnostic challenge in an African American male. Case: A 73-year-old African American male was admitted with altered mental status. Medical history included type 2 diabetes, hypertension, and hyperlipidemia. Home medications were carvedilol and simvastatin. On arrival, vital signs were normal. A fingerstick glucose was 52 mg/dL with a serum level of 68 mg/dL (70–110). Other labs were normal. Given symptomatic hypoglycemia, an IV dextrose infusion was initiated. Once his mentation improved, a diet was started. Despite this, he had recurrent hypoglycemia with glucose levels as low as 22 mg/dL, predominantly in fasting state with sporadic hyperglycemia. On rare occasions, he received correctional insulin for the same. An HbA1c was <4% (4–6). Thyroid function test and AM cortisol were normal. A cosyntropin stimulation test was negative for adrenal insufficiency. A hypoglycemia panel showed inappropriately high levels of insulin, highest at 77.4 μIU/mL(<=29.1), proinsulin of 19.7 pmol/L (<=8), and C-peptide of 5.6 (0.8–3.69 ng/mL) when serum glucose was 25 mg/dL. An MRI abdomen was normal. Octreotide study was negative for insulinoma. He had a normal response to IM glucagon, inferring normal glycogen stores. He was started on Diazoxide 160 mg thrice a day for recurrent hypoglycemia. An endoscopic ultrasound and DOTATATE scan were negative. He had no hypoglycemia for a few days, attributable to lingering effects of diazoxide. Eventually, his serum glucose was 52 mg/dL. Labs prior to glucose correction included an insulin level elevated at 1,000 (normal <3 mcIU/mL), c-peptide at 0.90 ng/mL, and proinsulin of 5.6 pmol/L. Given exceedingly high insulin levels, we measured an IAA level. This was >50 u/mL (normal <0.4 u/mL). With negative imaging and high IAAs, a diagnosis of IAS was made. Discussion: IAS or Hirata disease is a rare condition with hyperinsulinemic hypoglycemia and high titers of antibodies to endogenous insulin. The binding kinetics of endogenous insulin to these antibodies causes physiologically inappropriate levels of bioavailable insulin, causing either hyper- or hypoglycemia. IAA should be measured in patients with high insulin levels that are inconsistent with C peptide levels. We believe this to be the first African American patient to have been diagnosed with Hirata disease. Making a correct diagnosis may spare a hypoglycemic patient from unnecessary pancreatic surgical intervention.