Metabolic . Patient guide
Fasting Insulin
What is Fasting Insulin
Fasting insulin is the level of insulin in the blood after an overnight fast. Combined with fasting glucose it gives a far earlier signal of insulin resistance than HbA1c, often years before any glucose abnormality appears.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in pmol/L (also seen as mIU/L (divide pmol/L by 6.945)). Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Optimal (functional medicine view, not NICE-defined) | under 35 | pmol/L (equivalent to under 5 mIU/L); there is no NICE-defined optimal threshold for fasting insulin |
| Within typical lab range | 35 to 70 | |
| Suggests insulin resistance | 70 to 175 | |
| Significant insulin resistance likely | over 175 |
What it is
Insulin is produced by the beta cells of the pancreas in response to rising blood glucose. In insulin resistance, cells respond less efficiently and the pancreas compensates by producing more insulin to maintain a normal glucose. Fasting glucose can therefore stay normal for years while fasting insulin rises. The HOMA-IR calculation (fasting insulin × fasting glucose, divided by a constant) puts a number on the resistance state.
Why a clinician would order it
Fasting insulin is checked in metabolic risk assessment, in PCOS / PMOS work-up (high insulin drives ovarian androgen production), in unexplained weight gain particularly around the abdomen, in fatigue with afternoon energy slumps, and in proactive metabolic screening. Particularly useful in patients who have normal HbA1c but a family history of type 2 diabetes.
If your level is outside the range
Symptoms of low Fasting Insulin
- Symptoms attributable to genuinely low insulin are rare in non-diabetic patients
What low can indicate. Excellent insulin sensitivity (typical in athletes), low-carbohydrate diet, recent fasting or weight loss.
Symptoms of high Fasting Insulin
- Central weight gain
- Afternoon energy crash
- Sugar cravings
- Sleepiness after meals
- Acanthosis nigricans (dark velvety skin patches at neck or armpits)
- Skin tags
- Irregular periods or PCOS / PMOS symptoms in women
- Erectile changes in men
What high can indicate. Insulin resistance (the most common cause), early metabolic syndrome, PCOS / PMOS, obesity, sedentary lifestyle, certain medications (steroids, atypical antipsychotics), Cushing's syndrome, or rarely an insulinoma if extremely high.
Testing tips
Strict overnight fast (8 to 12 hours, water only) is essential. No coffee, no exercise, and ideally a normal carbohydrate intake the day before (not low-carb, which artificially lowers insulin). HOMA-IR is calculated from this result paired with fasting glucose. Hold metformin and other glucose-lowering medications only on clinician advice.
Where you can get this tested
Fasting Insulin is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Symptoms often investigated with Fasting Insulin
Fasting Insulin is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Related markers
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about Fasting Insulin
What is a normal Fasting Insulin range?
Optimal (functional medicine view, not NICE-defined): under 35 (pmol/L (equivalent to under 5 mIU/L); there is no NICE-defined optimal threshold for fasting insulin). Within typical lab range: 35 to 70. Suggests insulin resistance: 70 to 175. Significant insulin resistance likely: over 175. Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low Fasting Insulin result mean?
Excellent insulin sensitivity (typical in athletes), low-carbohydrate diet, recent fasting or weight loss.
What does a high Fasting Insulin result mean?
Insulin resistance (the most common cause), early metabolic syndrome, PCOS / PMOS, obesity, sedentary lifestyle, certain medications (steroids, atypical antipsychotics), Cushing's syndrome, or rarely an insulinoma if extremely high.
Do I need to fast or prepare for the Fasting Insulin blood test?
Strict overnight fast (8 to 12 hours, water only) is essential. No coffee, no exercise, and ideally a normal carbohydrate intake the day before (not low-carb, which artificially lowers insulin). HOMA-IR is calculated from this result paired with fasting glucose. Hold metformin and other glucose-lowering medications only on clinician advice.
Can I order a Fasting Insulin blood test privately in London?
Yes. WMG Health offers Fasting Insulin as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. Bespoke panels from £180; bookings via /contact/ or 020 3239 3378.