An elevated fasting C-peptide level is most commonly seen in which condition?

An elevated fasting C-peptide level is most commonly associated with conditions where there is an increased endogenous insulin production. In the case of insulinoma, which is a tumor of the pancreas that secretes insulin, elevated C-peptide levels indicate that there is an excess of insulin being produced. Since C-peptide is released in equal amounts with insulin when proinsulin is processed, higher levels of C-peptide reflect the tumor's activity in overproducing insulin, demonstrating the body's own heightened insulin synthesis rather than exogenous insulin administration.

In contrast, conditions such as Type 2 Diabetes can also lead to increased C-peptide levels but are more variable and may not always indicate elevated levels due to the presence of insulin resistance. Likewise, hypopituitarism typically results in low insulin production and corresponding low C-peptide levels, while pancreatitis may lead to varying levels of C-peptide depending on the nature of the pancreatic function at the time of assessment, but it is not primarily characterized by an increase in C-peptide. Thus, insulinoma stands out as the condition where elevated fasting C-peptide is most consistent due to enhanced insulin secretion.