Is a fasting insulin level of 2.5 microunits per milliliter (mU/mL) considered good in an adult patient with no known history of diabetes or metabolic disorders?

Is a Fasting Insulin Level of 2.5 mU/mL Good?

A fasting insulin level of 2.5 mU/mL is excellent and well within the normal range, indicating good insulin sensitivity and low risk for insulin resistance or metabolic disease.

Normal Reference Ranges

Your fasting insulin level of 2.5 mU/mL falls comfortably within established normal ranges:

• Population-based reference values from large studies show normal fasting insulin ranges from approximately 2-12 mU/mL for healthy adults 1
• A French study of over 4,000 individuals established reference values with a median of 5.1-5.6 mU/mL and 95th percentile of 11.2-12.6 mU/mL 2
• An Iranian population study found 95% reference values of 2.11-12.49 mU/mL 1

Your level of 2.5 mU/mL is at the lower end of normal, which is actually favorable and indicates excellent insulin sensitivity.

Clinical Interpretation for Insulin Resistance

According to the American College of Cardiology guidelines, fasting plasma insulin levels are interpreted as follows 3:

• Normal: <15 mU/L (your 2.5 mU/mL is well below this)
• Borderline high: 15-20 mU/L
• High: >20 mU/L (indicating insulin resistance)

Your value of 2.5 mU/mL suggests you have no insulin resistance and excellent metabolic health.

Why Low-Normal Insulin Is Favorable

A lower fasting insulin level within the normal range is associated with better metabolic outcomes:

• Lower cardiovascular risk: Elevated fasting insulin independently predicts coronary heart disease in patients with type 2 diabetes, even after controlling for other risk factors 4
• Lower diabetes risk: High fasting insulin predicts development of type 2 diabetes independent of insulin resistance 5
• Better insulin sensitivity: Lower fasting insulin at any given level of glucose indicates better insulin sensitivity 6

Important Context

Your fasting insulin should be interpreted alongside your fasting glucose:

• Normal fasting glucose is <100 mg/dL (5.6 mmol/L) 3, 7
• The combination of low-normal insulin with normal glucose indicates optimal metabolic function
• If your fasting glucose is also normal, this confirms excellent insulin sensitivity and low metabolic risk

Clinical Caveats

While your level is excellent, be aware that:

• Extremely low insulin (<3 mU/mL) with hypoglycemia (glucose <55 mg/dL) could suggest an insulinoma is unlikely, as these tumors typically cause inappropriately elevated insulin during hypoglycemia 8
• However, in the context of normal glucose levels, your value simply reflects good metabolic health
• Insulin levels should always be measured in the fasting state to avoid postprandial variations 3

In summary, your fasting insulin of 2.5 mU/mL is not just "good"—it's optimal, indicating excellent insulin sensitivity and low risk for metabolic disease.