Glucagon is the alpha-cell hormone of the pancreas. Counter-regulatory to insulin: when blood sugar drops, glucagon rises.
Mechanism — Stimulates hepatic glycogenolysis and gluconeogenesis, raising blood glucose. Also promotes fatty acid oxidation. Tirzepatide's third receptor target is the glucagon receptor — adding glucagon activity to GLP-1 + GIP for fat loss.
Lab range — Fasting glucagon 50-150 pg/mL. Rarely measured outside specific endocrine workups.
Caveats — Chronic insulin resistance often comes with elevated glucagon — the bi-hormonal disorder model of diabetes. Useful conceptually, less useful as a routine lab.