Insulin is a 51-amino-acid peptide hormone from pancreatic beta cells, released in response to elevated blood glucose.
Mechanism: Signals cells to take up glucose, suppresses hepatic glucose production, and drives anabolic processes. Lipogenesis, protein synthesis, glycogen storage.
Lab range: Fasting 2–20 µIU/mL standard; optimal under 8.
Caveats: Fasting insulin is one of the best early markers of metabolic dysfunction. It rises years before fasting glucose moves. HOMA-IR (insulin × glucose / 405) gives a single insulin-resistance number that's more diagnostic than either value alone.