What laboratory findings are indicative of insulinoma?

• A. Increased fasting glycemia and decreased C-peptide
• B. Decreased blood glucose and increased C-peptide
• C. Increased insulin and decreased C-peptide
• D. Normal blood glucose and normal C-peptide

Answer: (B)

In an insulinoma, which is a type of insulin-secreting tumor, you would expect to find decreased blood glucose levels due to the excessive insulin production by the tumor. This is a hallmark of insulinoma, as the tumor leads to inappropriate insulin secretion, ultimately causing hypoglycemia.

Additionally, there is an increase in C-peptide levels. C-peptide is released in equal amounts to insulin from the pancreatic beta cells, and its level reflects endogenous insulin production, as opposed to exogenous sources like injected insulin. Therefore, when an insulinoma is present, the ratio of insulin secretion to C-peptide remains intact and typically shows that the C-peptide levels are elevated due to the tumor's stimulation of insulin production.

The combination of decreased blood glucose and increased C-peptide is characteristic of endogenous hyperinsulinemia, such as that which occurs in patients with an insulinoma. This clinical picture is crucial for distinguishing insulinoma from other causes of hypoglycemia, particularly those caused by insulin administration, where C-peptide levels would typically be low.