What might decreased C-peptide and proinsulin levels in a patient with hypoglycemia indicate?

Decreased C-peptide and proinsulin levels in a patient experiencing hypoglycemia are indicative of surreptitious administration of exogenous insulin. This occurs because when exogenous insulin is injected or administered, it bypasses the physiological process that produces C-peptide. Normally, when proinsulin is converted to insulin in the pancreas, C-peptide is released in equal amounts. Therefore, if a patient has low levels of both C-peptide and proinsulin during hypoglycemic episodes, it suggests that the source of the insulin is not from the pancreas, but rather from an external source, such as an injected insulin.

In cases of insulinoma, which is a pancreatic tumor that secretes insulin, you would expect to find elevated levels of both insulin and C-peptide since the tumor produces endogenous insulin. Similarly, an adrenal insufficiency scenario might lead to hypoglycemia but would not typically present with decreased C-peptide or proinsulin levels related to insulin administration.

Hence, the critical takeaway is that low levels of C-peptide and proinsulin, alongside hypoglycemia, strongly point toward exogenous insulin use, confirming the diagnosis of exogenous administration as the cause of the hypoglycemic state in this context.