title: Beta-cell adenomas without hyperinsulinemia with use of highly specific insulin radioimmunoassays: case report and review of literature.
abstract: OBJECTIVE
  To report a case of a proinsulin-secreting islet cell adenoma in which the diagnosis was obscured by an ultraspecific insulin assay.
  
  
  METHODS
  We describe the case of a 46-year-old woman, who presented with fasting hypoglycemia and appropriately low insulin values.
  
  
  RESULTS
  A prolonged supervised fast produced symptomatic hypoglycemia (20 mg/dL) after only 7 hours. During the entire fasting test, highly specific insulin remained at <3 mIU/L, with a median value (and interquartile range) of 0.9 (0.8 to 2.3) mIU/L, when the glucose concentration was <50 mg/dL. The serum C-peptide level remained high normal (mean +/- SD, 2.7 +/- 0.6 ng/mL; normal fasting levels, 0.8 to 3.9), and no evidence of sulfonylurea use was detected in the patient's urine. Circulating proinsulin levels were persistently high (>200 pmol/L in all determinations when hypoglycemia was present; expected value, <5 pmol/L). Magnetic resonance imaging and endoscopic ultrasonography confirmed the presence of a 2.5-cm tumor in the head of the pancreas. A proinsulin-secreting islet cell tumor was diagnosed. Surgical resection of the tumor was successfully accomplished, but diabetes mellitus developed 4 months postoperatively.
  
  
  CONCLUSIONS
  The diagnosis of a hypoglycemia-producing pancreatic adenoma can be missed when an ultraspecific insulin assay is used. The direct measurement of proinsulin established the diagnosis in this case.
authors.affiliation: Department of Endocrinology and Metabolism, Instituto Nacional de Ciencias Médicas y Nutrición Salvador Zubirán, Mexico City, Mexico. fcojgomezp@aol.com
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Gómez-Pérez
authors.forename: Daniel
authors.initials: D
authors.lastname: Cuevas-Ramos
authors.forename: Paloma Almeda
authors.initials: PA
authors.lastname: Valdés
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Aguilar-Salinas
authors.forename: Roopa
authors.initials: R
authors.lastname: Mehta
authors.forename: Juan A
authors.initials: JA
authors.lastname: Rull
mesh_terms: D003937:Diagnosis, Differential; D019160:Endosonography; D005260:Female; D006801:Humans; D007003:Hypoglycemia / Q000097:blood; D007328:Insulin / Q000097:blood; D007340:Insulinoma / Q000097:blood* / Q000175:diagnosis* / Q000378:metabolism / Q000601:surgery; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D010190:Pancreatic Neoplasms / Q000097:blood* / Q000175:diagnosis* / Q000378:metabolism / Q000601:surgery; D011384:Proinsulin / Q000097:blood* / Q000378:metabolism; D011863:Radioimmunoassay
chemical_list: D007328:Insulin; D011384:Proinsulin