FGF21 and pancreatic insulinoma: In addition, the changes in the plasma FGF21 levels were positively correlated with the changes in insulin (r = 0.61, p = 0.048), proinsulin (r = 0.84, p = 0.001), C-peptide (r = 0.48, p = 0.137) and plasma glucose (r = 0.20, p = 0.558) in patients with insulinoma (Fig. 2).