Fasting insulin normal range: what your insulin level means

Quick answer: the fasting insulin normal range at Mayo Clinic Laboratories is 2.6 to 24.9 mcIU/mL (also written µIU/mL or mU/L), and it only applies after at least 8 hours without food. Insulin units convert to SI at about 6 pmol/L per mcIU/mL, so the same range is roughly 16 to 149 pmol/L. A result inside the range can still point to insulin resistance if your glucose is borderline, which is why labs and doctors pair it with fasting glucose.

Check insulin output without a lab visitSiPhox does not measure insulin. Its panels include C-peptide, which the pancreas releases in equal amounts, plus HbA1c. $124 a test plus $25 shipping.

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Fasting insulin normal range

Unlike glucose or HbA1c, fasting insulin has no diagnostic cutoffs from the American Diabetes Association; NIDDK's diagnosis table uses only HbA1c and glucose tests. What you get is the lab's reference interval, which describes the middle of a tested population.

Assays differ, so another lab may print a slightly different range. Compare repeat results from the same lab where you can.

What fasting insulin shows

Insulin is the hormone that moves glucose out of the blood and into cells. When cells respond poorly to it (insulin resistance), the pancreas makes more to keep glucose normal. Fasting insulin captures that extra effort at rest. MedlinePlus lists other names for the test: fasting insulin, insulin serum, and total and free insulin.

That is why fasting insulin can flag trouble before fasting glucose or HbA1c move: glucose stays in range while insulin climbs. NIDDK describes insulin resistance as the process that leads to prediabetes and type 2 diabetes when the pancreas can no longer keep up.

Fasting insulin and HOMA-IR

HOMA-IR combines fasting insulin and fasting glucose into one insulin-resistance score, based on the homeostasis model published by Matthews and colleagues in 1985. In US units: HOMA-IR = fasting glucose (mg/dL) x fasting insulin (mcIU/mL) / 405. In mmol/L, divide by 22.5 instead. Our HOMA-IR guide covers the cutoffs studies use and a calculator.

The chart makes the point: at the same normal glucose, a fasting insulin of 20 gives four times the HOMA-IR of an insulin of 5, yet both sit inside the lab's reference range.

High and low fasting insulin

MedlinePlus explains results in pairs with glucose:

• High insulin, normal or slightly high glucose: may mean insulin resistance.
• High or normal insulin, low glucose: may mean too much insulin, from an insulinoma (a pancreatic tumor), Cushing syndrome or taking too much insulin for diabetes.
• Low insulin, high glucose: the pancreas may not be making enough, as in type 1 diabetes or pancreatitis.

What lowers insulin resistance is the same list that lowers diabetes risk: weight loss if you carry extra weight, regular activity and a diet with fewer refined carbohydrates. Medication decisions belong with your doctor. If you already know you want an insulin-resistance workup, our at-home insulin resistance test guide compares options.

Worked example

Two 42-year-olds both have a fasting glucose of 96 mg/dL, comfortably normal. Person A has a fasting insulin of 6 mcIU/mL: HOMA-IR = 96 x 6 / 405 = 1.42. Person B has 18 mcIU/mL: HOMA-IR = 96 x 18 / 405 = 4.27. Both insulin values are inside the Mayo reference range of 2.6 to 24.9, but B's pancreas is producing three times as much insulin to hold the same glucose. B also has a triglyceride:HDL ratio of 3.5 (triglycerides 175, HDL 50), another marker that tends to rise with insulin resistance. B is the person to recheck in 3 to 6 months after lifestyle changes, and to discuss with a doctor.

Which test to use

How to prepare for the test

• Fast 8 to 12 hours, with nothing but water (MedlinePlus). Mayo's 2.6 to 24.9 range only applies after at least 8 hours.
• Stop biotin at least a day before, including hair, skin and nail supplements and multivitamins that contain it (MedlinePlus).
• Get glucose from the same draw. HOMA-IR needs fasting glucose and insulin from one sample; a glucose from another day gives a misleading score.
• If you take insulin, ask about C-peptide instead. MedlinePlus notes C-peptide is unaffected by insulin taken as medicine, so it shows what your own pancreas makes.
• Retest at the same lab after 3 to 6 months of changes. Assays differ, and one result can be moved by a poor night's sleep or a late snack.

Does SiPhox test fasting insulin?

No, not on its current EasyDraw panels. SiPhox's older finger-prick heart panel listed fasting insulin and HOMA-IR, but none of the six EasyDraw panels (Longevity Essentials, Heart & Metabolic, Thyroid Focus, Hormone Focus, Ultimate 360, GLP Optimization) includes them. The closest markers SiPhox does measure:

• C-peptide, on Longevity Essentials, Heart & Metabolic, Ultimate 360 and GLP Optimization. MedlinePlus explains that C-peptide and insulin enter the blood at the same time and in equal amounts, and C-peptide lasts longer, so it tracks insulin output.
• HbA1c and the triglyceride:HDL ratio, on the same panels.

C-peptide rises after meals, so a morning draw before eating gives the result closest to a fasting value. There is no doctor review included; bring a high result to your own doctor. See our SiPhox review for the full picture.

A note on home samples: SiPhox's partner lab tests capillary blood collected from the upper arm. SiPhox says its results show a 95% to 99% correlation with venous draws in its own quarterly comparisons against Labcorp and Quest, but it has not published a study of the EasyDraw device. If a result would change a treatment decision, confirm it with a venous test through your doctor.

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