Insulin

Insulin is the hormone your pancreas releases to move glucose out of the blood and into cells, and the fasting level reflects how hard the pancreas is working to keep blood sugar normal. A high fasting insulin often signals insulin resistance, the metabolic state that precedes type 2 diabetes and frequently accompanies central weight gain, fatty liver, and PCOS, and it commonly rises before fasting glucose or A1c move at all. Low insulin with low glucose points the other way, toward conditions like insulinoma, a rare insulin-secreting pancreatic tumor that causes hypoglycemia. The American Diabetes Association does not use insulin to diagnose diabetes, so this is best read as a metabolic-resistance and hypoglycemia marker rather than a diagnostic for diabetes itself.

This is the standalone fasting insulin measurement. If you also want to separate the insulin your body makes from injected insulin, the Insulin and C-Peptide Panel adds C-peptide, and Insulin Autoantibody is the test for suspected antibody interference; note that in people treated with insulin, binding antibodies can make a plain insulin result unreliable.