Insulin (Fasting) Test

A fasting insulin test measures the level of insulin — the hormone that regulates blood sugar by helping cells absorb glucose from the bloodstream — after an overnight fast.

In a healthy person, fasting insulin levels remain relatively low because there's little glucose entering the bloodstream from food during the fasting period, so the pancreas doesn't need to release much insulin.

When cells become resistant to insulin's effects — a condition called insulin resistance — the pancreas compensates by producing more insulin than normal to keep blood sugar in check, which shows up as an elevated fasting insulin level even before blood glucose itself becomes abnormal.

This makes fasting insulin, often combined with fasting glucose to calculate a HOMA-IR score, a useful early marker for metabolic problems that can precede a formal diabetes diagnosis by years.