The fasting insulin cutoff worth memorizing
You already order fasting glucose and A1c. The functional addition is fasting insulin, and it is one of the highest-yield labs in metabolic medicine because of what it catches and how early it catches it.
Why it matters
Insulin resistance does not start when glucose goes abnormal. It starts long before. The pancreas compensates by producing more and more insulin to keep glucose in range, and it can do that for ten to fifteen years before fasting glucose ever crosses 100. By the time glucose and A1c are abnormal, the dysfunction has been underway for a decade. Fasting insulin lets you see the compensation while it is still happening, which is exactly when intervention works best.
The number to memorize
A functional optimal fasting insulin is roughly below 5 to 6 microIU/mL. Hold that against what the lab tells you. Standard reference ranges run up to about 25, so a fasting insulin of 18 or 20 reads as normal even though it reflects significant resistance. That gap is the whole point. The lab’s normal and the functional optimal are not the same thing, and the space between them is where early insulin resistance lives, invisible on a standard report.
An honest caveat
There is no guideline-established cutoff for fasting insulin, and values vary by lab and assay. Treat below 5 to 6 as a functional optimal target to anchor your thinking, not a hard diagnostic line. A single number is a snapshot. The trend over time, and the company it keeps, triglycerides, HDL, waist circumference, the patient’s story, tell you what it means.
When to order it
Not a reflexive add-on for everyone. It earns its place when the clinical picture warrants it, the patient with central weight gain, fatigue, a family history of type 2 diabetes, PMOS, formerly known as PCOS, hypertension, or a triglyceride to HDL ratio that is creeping up, all with a normal glucose. Those are the patients in the silent compensation window.
The functional takeaway
Glucose tells you whether the system has already failed. Fasting insulin tells you how hard the system is working to keep from failing, years earlier. Memorize the functional optimal target, hold it against the lab’s permissive normal, and you catch metabolic dysfunction in the window where early intervention still changes the trajectory.
Written by Dr. Sheri Erwin, DNP, APRN, FNP-C
Founder, BridgeWell Integrative Education. 30+ years in healthcare, 16+ years training nurse practitioners. Systems-based, CE-accredited, and designed for NP scope from the ground up.
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