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Enzymatic process forms insulin in pancreatic secretory granules of beta cells. Proinsulin level is normally ≤20% of total insulin. Proinsulin has low biologic activity (10% insulin potency) and is the major storage form of insulin. Proinsulin is included in the immunoassay of total insulin, and separation requires special technique.
Normal range: 2.0–2.6 pmol/L.
Proinsulin: Insulin ratio is used as an indirect marker of beta cell function.
High proinsulin levels associated with benign or malignant beta cell tumors of the pancreas and endocrine pancreatic tumors associated with MEN-1.
Elevated levels are positive risk factor for the development of NIDDM.
Elevated levels in patients with chronic renal failure, cirrhosis, and hyperthyroidism.
Proinsulin >30% of serum insulin after overnight fast suggests insulinoma.
Proinsulin is increased in factitious hypoglycemia due to sulfonylurea.
Proinsulin is increased in familial hyperproinsulinemia—heterozygous mutation affecting cleavage of proinsulin leading to secretion of excess amounts of proinsulin.
Type 2 DM.
Proinsulin may also be increased in renal disease.
Elevation of proinsulin: insulin ratio correlates with a decreased acute illness response to glucose in patients with type 2 DM.