Plasma glucagon levels suppressed by a glucose load in a man with incidental pancreatic glucagonoma

Objective: To describe a patient with a histologically proven pancreatic glucagonoma, noted incidentally during a follow-up visit for high aminotransferase levels, and to evaluate its autonomy with a standard 75-g oral glucose tolerance test. Methods: We present the results of a 2-hour oral glucose tolerance test, with plasma glucagon and blood glucose levels measured every 30 minutes after an oral glucose load. In addition, we provide a brief review of the literature on the diagnosis and management of glucagonomas and the importance of long-term surveillance. Results: In our patient, who had a 1-year history of impaired fasting glucose, plasma glucagon levels were persistently suppressed to within the normal range after oral glucose challenge. Octreotide scintigraphy revealed abnormal uptake in the pancreatic tail, and a 2.8-cm mass was removed at laparoscopic distal pancreatectomy. Immunohistochemical staining of the tumor tissue showed intense reactivity for glucagon. Plasma glucagon levels were reduced to <50 pg/mL postoperatively, and scintigraphic study at 4-month follow-up showed no residual uptake at the previous tumor site or elsewhere. Conclusion: Glucagon-secreting pancreatic tumors are extremely rare. A substantially elevated plasma level of glucagon is usually seen in patients with metastatic tumors. In the early stage of a glucagonoma, however, the plasma glucagon level may be only modestly elevated and may still be susceptible to normal negative feedback inhibition. We demonstrated plasma glucagon complete suppressibility after oral glucose challenge in a patient with a glucagonoma, the first such report in the literature.