WHY DON'T DOCTORS TEST INSULIN LEVELS?

Millions of people in the UK are living with insulin resistance without knowing it. Blood sugar tests are a routine part of NHS health checks, but insulin itself is rarely measured, even when someone has symptoms that might point directly to a metabolic problem. If you have ever wondered why your GP has never checked your insulin, you are far from alone.

This article explains why insulin testing is not standard practice, why that matters, and what you can do if you want a clearer picture of your metabolic health. If you would like to check your levels privately, One Day Tests offers an insulin blood test with no GP referral needed and results typically available within 24 to 48 hours.

The short answer is that NHS blood testing follows clinical guidelines focused on diagnosing conditions rather than screening for early dysfunction. Insulin resistance, the stage where the body is struggling to respond to insulin but blood sugar has not yet risen to a diagnosable level, does not have an official diagnostic threshold in mainstream UK clinical guidance. Because there is no agreed point at which a GP is expected to act on a fasting insulin result alone, routine testing has not been built into standard care pathways.

This is not necessarily a failing of individual doctors. GPs work within structured protocols, and insulin testing is simply not included in the standard NHS health check or routine diabetes screening. Blood glucose and HbA1c, which reflects average blood sugar over roughly three months, are the markers used to identify prediabetes and type 2 diabetes. These are well-validated tools for diagnosing established disease. The problem is that by the time they show an abnormality, insulin resistance may have been present for years.

There are several other reasons why insulin testing tends to be overlooked.

Medical training has historically focused on treating diagnosed disease rather than identifying early metabolic dysfunction. The clinical significance of raised fasting insulin in someone with normal glucose is not always emphasised during training, which means some GPs are genuinely uncertain about how to interpret or act on the result.

NHS funding constraints also play a role. Insulin testing adds cost to an already stretched system, and without clear clinical guidelines mandating it, it is rarely prioritised. Some GPs may also be reluctant to order a test if they are uncertain what to do with a result that sits in a grey area.

Finally, there is a degree of pragmatism at play. Because insulin resistance is so common in the general population, some clinicians take the view that lifestyle advice applies broadly regardless of whether insulin is formally measured. While this is not unreasonable as a public health position, it means that individuals who could benefit from knowing their specific numbers often do not get the chance to find out.

Insulin is a hormone produced by the pancreas. Its primary job is to help glucose move from the bloodstream into the body's cells, where it can be used for energy or stored for later. When you eat carbohydrates, blood glucose rises, the pancreas releases insulin, and glucose is cleared from the blood. In a healthy metabolic system, this process is efficient and blood sugar returns to a normal range relatively quickly.

Insulin resistance develops when cells gradually become less responsive to insulin's signal. The pancreas compensates by producing more insulin to achieve the same effect. For a period of time, often years, this compensation works. Blood glucose stays within the normal range, HbA1c looks fine, and nothing in a standard blood test raises a flag. But underneath, insulin levels are climbing.

This is precisely the window that a fasting insulin test can detect. By measuring how much insulin is circulating in the blood after an overnight fast, it reveals how hard the pancreas is working to maintain normal blood sugar. A raised fasting insulin with a normal glucose level is a meaningful finding. It suggests the body is compensating, and that the compensation may not hold indefinitely.

Eventually, if insulin resistance continues to worsen, the pancreas can no longer keep up. Glucose begins to rise, and prediabetes or type 2 diabetes is diagnosed. At that point, significant metabolic damage has often already occurred. Research suggests insulin resistance can precede a type 2 diabetes diagnosis by a decade or more.

It helps to understand what each test actually tells you, because they measure different things.

Fasting glucose gives a snapshot of blood sugar at a single point in time, after a period without food. It is useful but limited, because the body can keep glucose looking normal for a long time even as insulin resistance progresses, simply by producing more insulin.

HbA1c reflects average blood glucose over the past two to three months. It is a more stable indicator than a single glucose reading and is the main tool used to diagnose prediabetes and type 2 diabetes in the UK. But like fasting glucose, it only tells you what glucose is doing. It says nothing about how much insulin is required to keep glucose at that level.

Fasting insulin fills that gap. It shows whether the system is working efficiently or whether the pancreas is under strain. Someone can have completely normal glucose and HbA1c values and still have significantly elevated fasting insulin, which may indicate early insulin resistance that warrants attention.

For a more complete picture, fasting insulin is sometimes measured alongside fasting glucose to calculate HOMA-IR, which stands for Homeostatic Model Assessment of Insulin Resistance. This is a simple calculation that gives a numerical estimate of insulin resistance. A HOMA-IR above 2.0 is generally considered a marker of early insulin resistance, though interpretation should always take clinical context into account.

Fasting insulin testing is not only relevant to people who already have diabetes. It may be worth considering if you have any of the following:

Persistent difficulty losing weight, particularly around the abdomen, despite making dietary changes. Insulin is a fat-storing hormone, and when levels are chronically elevated it makes it harder for the body to burn stored fat.

Fatigue after meals, energy dips in the afternoon, or cravings for sugar or starchy foods. These can all be signs that blood sugar and insulin are fluctuating more than they should.

A family history of type 2 diabetes, heart disease, or polycystic ovary syndrome (PCOS). Insulin resistance has a hereditary component and tends to run in families.

PCOS itself. Raised insulin is a central driver of PCOS in many women, contributing to irregular cycles, elevated androgens, and difficulty managing weight. Testing insulin can be an important part of understanding what is driving the condition.

Borderline blood pressure or elevated triglycerides, even within the technically normal range. These can be early signs of metabolic strain and often accompany raised insulin.

A sense that something is off with your energy, weight, or metabolism, even though standard blood tests have come back normal. This is one of the most common frustrations people raise, and it is exactly the situation where fasting insulin can provide useful additional information.

You can ask your GP to test your fasting insulin, though it is not always straightforward. Because it is not part of routine screening, some GPs will only order it if there is an established clinical reason, such as a confirmed diabetes diagnosis or investigation for PCOS. If your blood glucose and HbA1c are normal, a request for insulin testing may not always be fulfilled on the NHS.

Privately testing your fasting insulin is a practical alternative. You do not need a GP referral, the test is straightforward, and it requires only a standard fasting blood sample taken after eight to twelve hours without food, ideally in the morning.

When interpreting results, it is worth knowing that the standard laboratory reference range for fasting insulin is broad, and that a result within the reference range does not necessarily mean insulin levels are optimal. Many practitioners in preventive medicine consider a fasting insulin below 7 pmol/L to represent good insulin sensitivity, while values above this, even within the formal reference range, may still be worth discussing with a healthcare professional.

If your result comes back raised, it does not mean you have diabetes or are inevitably heading towards it. Insulin resistance at this stage is often highly responsive to lifestyle changes, including dietary adjustments, regular physical activity, improved sleep, and stress management. The value of knowing your result is that it gives you something specific to act on.