Fasting Insulin

What it is

Fasting insulin refers to the concentration of insulin in the blood measured after an overnight fast. It can also be measured at multiple timepoints during an OGTT. Insulin is the hormone produced by the pancreas that allows cells to absorb glucose from the bloodstream.

In PCOS, insulin resistance (IR) is present in 50 to 70% of individuals, regardless of weight. Measuring fasting insulin is one way to evaluate this resistance, though its limitations must be understood.

Normal values and insulin resistance thresholds

Reference values vary between laboratories because there is no international standardisation. As a general guide:

• Normal: < 10 mU/L (also expressed as µU/mL)
• Probable insulin resistance: > 12 – 15 mU/L fasting
• Severe insulin resistance: T60 peak > 150 mU/L during an OGTT

These thresholds are indicative. Fasting insulin alone is not sufficient to diagnose insulin resistance — it is combined with fasting glucose to calculate HOMA-IR.

Relationship with HOMA-IR

HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) is the most widely used index in clinical practice to quantify insulin resistance. It is calculated as:

HOMA-IR = (fasting insulin mU/L × fasting glucose mmol/L) ÷ 22.5

A HOMA-IR > 2.5 is considered a threshold for insulin resistance in many PCOS studies, although this threshold varies across study populations.

Why fasting insulin is not measured routinely

Unlike blood glucose, fasting insulin is not standardised across laboratories: measurement methods differ, making comparisons difficult. This is why it is not part of routine blood panels and remains a specialised test, often requested as part of a comprehensive PCOS workup.

Insulin levels are also highly sensitive to conditions at the time of sampling (stress, recent physical activity, timing of the previous meal). An isolated value can be difficult to interpret without clinical context.

Post-glucose insulin

During an OGTT, insulin measured at T60 minutes is particularly informative. A peak > 150 mU/L at 1 hour suggests reactive hyperinsulinaemia — a sign of insulin resistance even when glucose remains normal. This information helps guide treatment decisions regarding metformin or inositols.

Key takeaways

• Normal fasting insulin: < 10 mU/L
• Probable resistance if > 12 – 15 mU/L fasting
• Combined with fasting glucose to calculate HOMA-IR
• Not standardised between laboratories — interpret in context
• T60 peak > 150 mU/L during an OGTT indicates severe resistance