Insulin

Overview

Insulin is a peptide hormone secreted by pancreatic beta cells that regulates carbohydrate, fat, and protein metabolism by promoting glucose uptake into muscle and adipose tissue and suppressing hepatic glucose production. It is a key determinant of fasting and postprandial blood glucose concentrations and reflects beta‑cell function and insulin sensitivity. Low fasting levels can indicate insulin deficiency (type 1 diabetes, late‑stage type 2 diabetes), whereas high fasting or post‑challenge levels often reflect insulin resistance and compensatory hyperinsulinemia. Insulin measurement is clinically useful for diagnosing hypoglycemia, assessing insulin resistance, and classifying diabetes subtypes, although it is used more selectively than glucose or HbA1c.

Clinical Use Cases

  • Investigating fasting hypoglycemia (e.g., endogenous hyperinsulinemia vs surreptitious insulin use).
  • Assessing insulin resistance and early metabolic syndrome (e.g., with glucose in fasting and oral glucose tolerance tests).
  • Differentiating type 1 from type 2 diabetes or latent autoimmune diabetes in adults (LADA).
  • Monitoring beta‑cell function in insulinoma and other rare endocrine disorders.

Specimen Types

  • Serum.
  • Plasma (usually EDTA or lithium heparin, processed promptly).

Measurement Methods

  • Immunoassay (chemiluminescent or radioimmunoassay).
  • Enzyme‑linked immunosorbent assay (ELISA) in some research or reference settings.
  • Automated immunoanalyzers commonly used in clinical laboratories.

Test Preparation and Influencing Factors

  • Fasting state is often required for baseline interpretation (usually 8–12 hours).
  • Recent food intake, exercise, stress, or glucose load markedly elevate insulin levels.
  • Insulin‑sensitizing drugs (e.g., metformin, thiazolidinediones) and weight loss may lower insulin, whereas glucocorticoids and obesity often increase it.
  • Hemolysis, lipemia, and delays in sample separation can affect results.

Synonyms

  • Serum insulin.
  • Fasting insulin (when measured in fasting state).
  • Immunoreactive insulin (older term for insulin‑like immunoreactivity).

Further Reading