What this test measures
Insulin is the principal anabolic hormone produced by the beta-cells of the pancreas. After an overnight fast, healthy beta-cells secrete a low baseline level of insulin sufficient to keep fasting glucose in range. The fasting insulin level reflects this baseline output — and, paired with fasting glucose, gives a numerical estimate of insulin sensitivity through the HOMA-IR index (fasting insulin × fasting glucose ÷ 405 in conventional units).
The test is run after an 8–10 hour fast. Modern immunoassays measure intact human insulin without significant cross-reaction with proinsulin or C-peptide.
Why it matters
Indians are particularly prone to insulin resistance — the "Indian phenotype" of central obesity, low HDL and high triglycerides with normal BMI is well-described. Fasting insulin with HOMA-IR is one of the most useful early markers of insulin resistance — often raised years before fasting glucose or HbA1c become abnormal. This early signal matters because lifestyle interventions are most effective in the prediabetic stage.
Fasting insulin is also used in the workup of recurrent hypoglycaemia (suspected insulinoma — the classic "Whipple's triad" with high insulin during documented hypoglycaemia and inappropriately preserved C-peptide), in PCOS workup (most women with PCOS have insulin resistance), and to characterise metabolic syndrome.
How to prepare
Fast for 8–10 hours before the test (water allowed). No vigorous exercise the morning of the test. Stop biotin for 48–72 hours. If you are on insulin or insulin secretagogues (sulfonylureas, repaglinide), tell your doctor — these directly affect the result.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Fasting Insulin (µIU/mL)[1][2] | 2 – 25 µIU/mL (HOMA-IR < 2.0 — insulin sensitive) | Very low fasting insulin (< 3) with high fasting glucose suggests type 1 diabetes or late-stage type 2 with beta-cell exhaustion. Low insulin during documented hypoglycaemia rules out endogenous hyperinsulinism — consider exogenous insulin abuse (if C-peptide is low) or non-insulin causes. | Raised fasting insulin (> 12–15) with normal glucose = insulin resistance. HOMA-IR > 2.0 supports it; > 3 strongly suggests it. Persistently high insulin during documented hypoglycaemia with preserved C-peptide = endogenous hyperinsulinism (insulinoma). |
Fasting insulin + HOMA-IR — interpretation
| Fasting insulin (µIU/mL) | Fasting glucose (mg/dL) | HOMA-IR | Interpretation |
|---|---|---|---|
| < 10 | < 100 | < 2 | Insulin sensitive — normal |
| 10 – 15 | < 100 | 2 – 3 | Borderline insulin resistance — lifestyle focus |
| > 15 | < 100 | > 3 | Insulin resistant — high risk of progression to diabetes |
| > 15 | 100 – 125 | > 3.5 | Prediabetic insulin resistance |
| Very high | > 126 | Very high | Early T2DM with beta-cell still compensating |
| Low | > 126 | Variable | Late T2DM with beta-cell exhaustion, or type 1 DM |
| High during hypo | < 55 | — | Endogenous hyperinsulinism — workup for insulinoma |
Frequently asked questions
Do I need to fast?
Yes — 8 to 10 hours, water only. Eating any food in the few hours before testing causes insulin to spike, making the fasting result uninterpretable.
What is HOMA-IR?
Homeostatic Model Assessment of Insulin Resistance — a simple formula combining fasting glucose and insulin to estimate insulin sensitivity. A score above 2.0 suggests insulin resistance; above 3.0 is strongly suggestive.
My fasting glucose is normal but insulin is high — am I diabetic?
Not yet — but a raised fasting insulin with normal glucose is the classic biochemistry of "early insulin resistance," often years before diabetes develops. It is an early-warning result and a strong reason to act on lifestyle (weight, diet, exercise).
I am on metformin / GLP-1 agonists — does that affect the test?
Metformin and GLP-1 agonists do not raise insulin and broadly preserve interpretation. Sulfonylureas and exogenous insulin do — discuss timing with your doctor.
My insulin is high in PCOS — should I be on metformin?
Many women with PCOS have biochemical insulin resistance. Metformin can help cycle regularity and metabolic markers in selected patients, but is not universally recommended — your gynaecologist or endocrinologist will judge.
When is insulinoma suspected?
Recurrent hypoglycaemia (especially in fasting or after exercise) with documented low glucose, inappropriately high insulin and preserved C-peptide raises strong suspicion of an insulinoma. A 72-hour fast in a hospital is the diagnostic test.
Will the result be lower if I exercise the morning before?
Yes — exercise increases insulin sensitivity and lowers fasting insulin. Avoid vigorous exercise the morning of testing for a clean baseline.
Related Hormones / Endocrine tests
Tests commonly ordered alongside INSULIN - FASTING, or that help interpret an unexpected result.
Sources & references
- ADA Standards of Care in Diabetes 2025 · accessed 2026-05-30T00:00:00.000Z
- Endocrine Society — Hypoglycemia in Adults · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Insulin Test · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Insulin · accessed 2026-05-30T00:00:00.000Z
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