Background: Investigation of non-diabetic hypoglycaemia often involves a 72 hour fast. According to Endocrine Society guidelines, an increase in plasma glucose of at least 1.4mmol/L following IV glucagon administration at the conclusion of the fast indicates mediation of hypoglycaemia by insulin. We report the case of a patient with a functioning insulinoma, and a normal 72hr fast, who developed hypoglycaemia after glucagon-induced hypersecretion of insulin.
Case: A 57 year old lady presented after 3 episodes of post-prandial hypoglycaemia. No hypoglycaemia was observed after a 72hr fast, with a final plasma glucose level of 4.2mmol/L, and serum insulin and c-peptide levels of 8mIU/L and 0.59nmol/L respectively. IV Glucagon administration increased insulin and C-peptide levels to 5341mIU/L and 45.70nmol/L respectively at 10 minutes, with a subsequent decrease in plasma glucose levels to 2.3mmol/L at 30 minutes and 1.2mmol/L at 70 minutes when insulin and C-peptide levels were 1011mIU/L and 10.5nmol/L respectively. Computed tomography revealed a pancreatic body hypervascular mass. Selective arterial calcium stimulation confirmed a biochemically functional tumour. Resection of the mass revealed a neuroendocrine tumour with immunohistochemistry consistent with insulinoma. No further hypoglycaemic episodes occurred post-operatively.
Discussion: The 72hr fast identifies over 90% of insulinomas, except in patients with predominantly post-prandial symptoms. IV glucagon administration is recommended at the conclusion of the fast to help distinguish between insulin- and non-insulin-mediated hypoglycaemia. However, glucagon occasionally stimulates insulin secretion by insulinomas, with subsequent hypoglycaemia. This risk of hypoglycaemia is not identified in the Endocrine Society guidelines, which advise monitoring of plasma glucose for 30 minutes following glucagon administration. On the basis of our experience, we would suggest monitoring of plasma glucose for 60-70 minutes post-glucagon administration.
Reference: Cryer et al. Evaluation and Management of Adult Hypoglycemia J Clin Endocrinol Metab, March 2009, 94(3):709–728