Insulin

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Subject: Insulin

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Definition

  • This peptide hormone is enzymatically processed from proinsulin in pancreatic secretory granules of beta cells. Approximately 50% is removed from the blood during initial passage through the liver. Its half-life is 4–9 minutes. Secretion is regulated primarily by blood glucose levels; therefore, it should always be measured with concomitant blood glucose. Insulin deficiency is the crucial factor in the pathogenesis of type 1 DM.

  • Normal range: 6–27 μIU/mL.

Use

  • Diagnosis of insulinoma

  • Diagnosis of fasting hypoglycemia

  • Not clinically useful for diagnosis of DM

Interpretation

Increased In

  • Insulinoma. Fasting blood insulin level >50 μU/mL in the presence of low or normal blood glucose level. Administration of tolbutamide or leucine causes a rapid rise of blood insulin to very high levels within a few minutes, with a rapid return to normal.

  • Factitious hypoglycemia in the presence of normal blood glucose.

  • Insulin autoimmune syndrome.

  • Untreated mild DM in obese individuals. The fasting blood level is often increased.

  • Cirrhosis due to insufficient clearance from blood.

  • Acromegaly (especially with active disease) after ingestion of glucose.

  • Reactive hypoglycemia after glucose ingestion, particularly with the diabetic type of glucose tolerance curve.

Decreased In

  • Type 1 DM.

  • Hypopituitarism.

  • Severe DM with ketosis and weight loss, which may result in an absence of insulin. In less severe cases, insulin is frequently present but only at lower glucose concentrations.

Limitations

  • Insulin values are normal in

    • Hypoglycemia associated with nonpancreatic tumors

    • Idiopathic hypoglycemia of childhood, except after administration of leucine

  • Circulating anti-insulin antibodies are often found in patients who have been treated with nonhuman forms of insulin. If present, these antibodies may interfere with the assay.

  • For individuals who are significantly overweight, fasting insulin levels are typically somewhat higher than for adults of normal weight.

  • Heterophilic antibodies in human serum can react with the immunoglobulins included in the assay components, causing interference with in vitro immunoassays. Samples from patients routinely exposed to animals or animal serum products can demonstrate this type of interference, potentially causing an anomalous result.

 
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