Fasting Insulin
Insulin is the hormone that controls your blood sugar. You are probably hoping for a clear "is my number OK?", and the honest answer is there is no single agreed normal range. A high fasting level can be an early sign your body is needing more and more insulin to do the same job — but it is a prompt for a chat with your GP, not a diagnosis.
Targeted TestingWhat is fasting insulin?
Fasting insulin is the concentration of insulin — a hormone made by the pancreas — circulating in your bloodstream after you have gone without food for at least 8 hours, typically overnight. Insulin helps control blood glucose levels and plays a role in controlling the levels of carbohydrates and fats stored in the body. When you eat, blood glucose rises and the pancreas releases insulin to shuttle that glucose into your cells for energy or storage. The fasting sample removes the noise of recent meals so the reading reflects your baseline — how much insulin your pancreas must produce simply to maintain normal glucose at rest. Unlike a fasting glucose test, which measures the sugar itself, fasting insulin reveals how hard the pancreas is working. Elevated fasting insulin with a normal glucose can be an early sign of insulin resistance — a state in which the body has built up a tolerance to the actions of insulin, and insulin is less effective in doing its job; as a result, more insulin may be required to coax the cells and liver to take up the glucose from the blood.
Why is it tested?
A fasting insulin test is ordered to help assess insulin resistance — a condition in which the cells of the body do not respond to insulin effectively. Insulin resistance often develops years before fasting glucose climbs into the pre-diabetes range, so it can surface a metabolic signal that glucose testing alone would miss. At the same time, it is important to be honest about the test's limitations. According to the Royal College of Pathologists of Australasia (RCPA), measuring fasting insulin "may be performed to investigate insulin resistance but there is limited evidence for this purpose or agreement among health professionals on this indication." The Endocrine Society of Australia — through the Choosing Wisely Australia programme — advises not to routinely measure insulin in the fasting state to assess insulin sensitivity, noting that it is "not a clinically useful method (and may be costly because of the insulin assay) to estimate insulin sensitivity." The RCPA notes that "other indicators of insulin resistance such as waist circumference and lipid levels are more well established." Fasting insulin is therefore most useful in a targeted context: people with risk factors for insulin resistance or metabolic syndrome (a cluster of conditions including raised blood pressure, high triglycerides, low HDL cholesterol, and central weight gain), or people interested in understanding metabolic health in more depth than glucose alone provides. It is not a standard screening test.
Reference range (Australia)
Reported in mU/L (= mIU/L) by Australian labs including Sonic; the interval varies by laboratory and assay, so the range on your report is authoritative.
Reference intervals vary between laboratories. Always use the range printed on your own report.
What your result means
A lower-than-range fasting insulin is uncommon. It can occur in people who are lean and highly insulin-sensitive, meaning their cells respond to insulin very efficiently so the pancreas produces little at rest. Very low fasting insulin is not a concern on its own if your blood glucose is normal. In rare circumstances, very low insulin alongside raised glucose can point to conditions affecting the pancreas's ability to produce insulin — your GP can evaluate this in context of your full picture.
A fasting insulin within the range on your report suggests your pancreas is producing an appropriate amount of insulin at rest and your cells are responding normally. A normal result does not rule out all metabolic risk on its own — factors such as family history, weight, activity levels, and trends over time all matter. If risk factors are present, your GP may still suggest looking at fasting glucose, HbA1c, or lipid levels for a fuller picture of your metabolic health.
A higher-than-range fasting insulin does not mean you have diabetes, and your blood glucose may still be entirely normal. It suggests your pancreas is producing more insulin than usual to keep glucose steady, a pattern that can be consistent with insulin resistance (where the body has built up a tolerance to insulin, so the pancreas compensates by making more). It points to a modifiable metabolic risk factor, not a diagnosis. Bring the result to your GP, who will read it alongside your fasting glucose, HbA1c (your average blood sugar over the past 2 to 3 months), lipid profile and overall risk before deciding whether anything is worth doing.
What can affect your result
Physical activity has a strong influence: exercise, particularly resistance training and higher-intensity work, is associated with muscle cells taking up glucose with less insulin (doctors call this increased insulin sensitivity). Eating patterns higher in fibre and lower in refined carbohydrate and added sugar are associated with lower fasting insulin over time. Excess weight, particularly around the abdomen, is associated with reduced insulin sensitivity and higher insulin production; in people with overweight, a 5 to 10 per cent reduction in body weight is associated with meaningful improvement in fasting insulin. Sleep and stress also play a part, and your GP can weigh up which of these is relevant for you. On the medication side, corticosteroids, some thiazide diuretics, and certain antipsychotic medications can raise insulin levels. If you take regular medication, let your GP know when interpreting your result. Age, sex, and genetics influence baseline insulin production — factors that cannot be modified but are important context when your GP interprets the result.
When to act
If fasting insulin is raised, your GP will look at it alongside your fasting glucose, HbA1c, lipids, blood pressure and waist measurement to judge whether insulin resistance is present. Insulin sensitivity is influenced by activity, diet, weight, sleep and stress, and your GP or an accredited practising dietitian can talk through what, if anything, is worth changing for you. If you are retesting to track changes over time, allow at least 8 to 12 weeks between tests and fast properly beforehand, because eating or drinking anything other than water will raise both insulin and glucose and make the result hard to interpret. This page is general information and is not a substitute for advice from your GP.
Frequently asked questions
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