Fasting insulin: what it is, what it tells you and why we test it

Insulin is the hormone that lets sugar into your cells. Measured after a night without food, it shows how hard your pancreas is working to keep your blood sugar normal. It often rises years before glucose does.

Why we test it

After a meal your blood sugar rises, and your pancreas releases insulin to move it into your cells. If your cells respond less, your pancreas makes more insulin to get the same effect.

So your blood sugar can stay normal for years while your insulin is already high. This is called insulin resistance, and it often comes long before type two diabetes.

Glucose alone often misses it. Fasting insulin can show it earlier: that is why we test it.

What it tells you, and what it does not

If it is high while your glucose and HbA1c are normal, your pancreas is usually making up for insulin resistance: it keeps up, but works harder.

If it is low with a normal glucose, that is usually a good sign: your cells respond well to insulin. With a high glucose, though, it reads differently.

On its own it is not enough for a diagnosis: it changes a lot from day to day and with what you have eaten.

That is why it is read together with other tests:

Normal fasting insulin levels

The lab decides what is normal. Many lab reports give 2.6 to 24.9 µIU/mL. Your own report may print different limits, because not every test method gives the same number.

It is a wide range. Insulin resistance can already be there with a number inside your report’s limits, especially in the upper part.

Some reports write mIU/L: it is the same number. If yours gives it in pmol/L, the number is different and cannot be compared with this page.

In the Rosa report you also see your optimal range, read together with your glucose and HbA1c.

What your result means

Find the row for your fasting result, in µIU/mL. Insulin is always read together with glucose: if your glucose or HbA1c is high, talk to your doctor.

If your insulin is high or low

The HOMA index: what it is and how it is calculated

The HOMA index, or HOMA-IR, estimates insulin resistance from two results of the same fasting blood test: insulin and glucose. Some labs already print it on the report.

This is how it is calculated: insulin in µIU/mL times glucose in mg/dL, divided by 405. If your glucose is in mmol/L, divide by 22.5 instead.

The higher it is, the harder your body finds it to use insulin. But no limit fits everyone: it varies with the lab’s method and the population. That is why it is read with your glucose and your doctor.

What can skew the result

A few things change the number a lot. It helps to know them before your blood test.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal fasting insulin levels? Many lab reports give 2.6 to 24.9 µIU/mL, but the lab decides. Insulin resistance can already be there inside these limits: that is why the HOMA index is checked too, with glucose.

What is a normal HOMA index? There is no single limit: it varies with the lab and the population. Usually, the lower it is, the better your body uses insulin. Read it with your doctor, together with glucose and HbA1c.

Low insulin: what does it mean? With a normal glucose, it usually means your cells respond well to insulin, which is a good sign. If your glucose is high, though, your pancreas may be making too little: talk to your doctor.

Do I need to fast for an insulin test? Yes, and it matters even more than for glucose: even a small snack pushes it up a lot. You are usually asked not to eat from the evening before. You can drink water.

What is the difference between insulin and glucose? Glucose is the sugar in your blood. Insulin is the hormone that lets it into your cells. When your body responds less to insulin, insulin often rises years before glucose does.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it once a year. If it is high and you change your habits, it is usually checked again after about three months, or when your doctor says so.

Sources