Insulin Production: The Pancreas Is the Primary Source

Insulin is produced almost exclusively by pancreatic β-cells in the islets of Langerhans. These cells sense blood glucose via GLUT2/glucokinase and secrete insulin in a pulsatile, glucose-dependent manner.

Extrapancreatic insulin production is negligible in humans. While insulin mRNA has been detected in thymus, brain, and fetal tissues, and some studies suggest local synthesis in adipose or intestinal cells under specific conditions, these sources do not contribute meaningfully to circulating insulin levels or glucose homeostasis in adults.

Liver and kidney roles:

• Liver: Major site of insulin clearance (first-pass extraction ~50%); produces IGF-1 (insulin-like growth factor) under GH stimulation.
• Kidney: Degrades filtered insulin; produces erythropoietin, activates vitamin D.
• Adipose tissue: Secretes adipokines (leptin, adiponectin) influencing insulin sensitivity.

Thus, the pancreas is the sole physiologically relevant source of circulating insulin. β-cell failure → diabetes mellitus.