Fasting insulin measures insulin levels after an overnight fast, when the pancreas should be producing minimal insulin in the absence of dietary glucose. In people with normal insulin sensitivity, fasting insulin is low. In insulin resistance, the pancreas compensates by producing more insulin to maintain blood glucose in range — so fasting insulin rises before fasting glucose becomes abnormal.

This makes fasting insulin one of the earliest detectable markers of metabolic dysfunction — often elevated for years before pre-diabetes appears on standard glucose tests. Combined with fasting glucose, fasting insulin is used to calculate HOMA-IR, a measure of insulin resistance. Elevated fasting insulin is also linked to obesity, PCOS, cardiovascular risk, and certain cancers.