A patient gets an annual physical. Fasting glucose comes back at 94 mg/dL, comfortably inside the normal range. A1c is 5.4%, also normal. The doctor moves on. What almost never gets checked in that same visit is fasting insulin, and that's the number that would have told a very different story years earlier.
Glucose and insulin don't rise together. Insulin rises first, sometimes by over a decade, while your pancreas works harder and harder to keep glucose looking normal. By the time glucose itself moves, the compensation has usually been running for years.
Why Glucose Stays Normal Long After Insulin Doesn't
Your pancreas doesn't let glucose rise the moment your cells start resisting insulin's signal. It compensates, producing more insulin to force the same amount of glucose into your cells. That compensation is effective, and it's also invisible on a standard glucose panel. Glucose stays in range specifically because insulin is working overtime to keep it there.
A study following 6,538 adults without diabetes for up to 13 years captured this directly. Researchers tracked insulin sensitivity and beta-cell function retrospectively in everyone who went on to develop type 2 diabetes. Insulin sensitivity and beta-cell compensation began shifting measurably years before diagnosis. Fasting glucose itself only rose abruptly in the final year or two before diagnosis (Tabák AG et al., The Lancet, 2009 — PMID: 19515410).
Signs That Should Prompt a Fasting Insulin Check
Fasting insulin isn't part of a standard panel, so it usually only gets ordered if someone thinks to ask for it. A few other lab findings and physical signs are strongly enough linked to insulin resistance that they're worth treating as a prompt to check insulin directly, rather than something to note and move past.
Low SHBG
Sex hormone binding globulin is made in the liver, and insulin suppresses its production. A population survey of 2,782 adults found a significant inverse relationship between fasting insulin and SHBG in both men and women, independent of age and BMI (Daka B et al., Endocrine Connections, 2012 — PMID: 23781314). A low SHBG on a routine hormone panel is a reasonable trigger to check fasting insulin directly, not just a number to note in passing.
Low HDL and a High Triglyceride-to-HDL Ratio
Insulin resistance changes how the liver handles fat, and that shows up in a specific lipid pattern: triglycerides rise and HDL falls. A study of 258 overweight adults found that both triglyceride level and the triglyceride-to-HDL ratio independently identified people who were insulin resistant, with accuracy comparable to the standard metabolic syndrome criteria (McLaughlin T et al., Annals of Internal Medicine, 2003 — PMID: 14623617). This ratio isn't equally reliable across every population; it performs less consistently in some ethnic groups, particularly African American women, so it's one data point among several, not a standalone verdict.
An Inverted LH-to-FSH Ratio, in Women
This one applies specifically to women. An elevated LH-to-FSH ratio is a recognized feature of polyendocrine metabolic ovarian syndrome (PMOS, the condition formerly known as PCOS), and the majority of women with PMOS have insulin resistance, with estimates ranging from 50 to 80 percent. If you see the classic elevated LH-to-FSH ratio, insulin resistance is worth investigating directly. This is a screening signal that insulin resistance is likely, not a tool for measuring how severe it is.
Acanthosis Nigricans
This is a physical exam finding, not a lab value: velvety, darkened skin, most often at the back of the neck, armpits, or knuckles. A study of adults aged 40 to 60 found it to be a highly prevalent and specific clinical sign of insulin resistance in that age group (Álvarez-Villalobos NA et al., International Journal of Clinical Practice, 2020 — PMID: 31769902). It's one of the few insulin-resistance markers you can actually see without ordering anything.
A Familiar Pattern
A patient in their forties came in for a routine evaluation with fasting glucose and A1c both comfortably normal, but with fatigue after meals, stubborn midsection weight despite a consistent training routine, and a family history of type 2 diabetes on both sides. Fasting insulin came back elevated. A HOMA-IR calculation confirmed meaningful insulin resistance sitting underneath two "normal" standard labs. That finding changed the actual conversation, from reassurance based on glucose alone to an early intervention plan based on what was really happening.
Who Should Have This Conversation
This is relevant if you have a family history of type 2 diabetes, midsection weight that hasn't responded to consistent effort, fatigue that tends to hit after meals, or any of the markers above on a recent panel or exam, even with normal glucose and a normal A1c.
What a Realistic Next Step Looks Like
Fasting insulin, paired with fasting glucose, allows a HOMA-IR calculation, a more complete picture than either number alone. This isn't a one-time test and done; if insulin resistance is found, follow-up testing tracks whether an intervention is actually working, well before glucose itself would show any change.
A normal glucose panel isn't the same thing as a clean bill of metabolic health. It just means the compensation is still working.
Has anyone ever actually checked your fasting insulin, or just your glucose?
A free consultation is a good place to start that conversation, no commitment required. To request one, email drwilcox@precisionhormoneconsulting.com.
— Darrell Wilcox, MD | @wellnessdoc_4everyoung on Instagram
References
Tabák AG, Jokela M, Akbaraly TN, Brunner EJ, Kivimäki M, Witte DR. Trajectories of Glycaemia, Insulin Sensitivity, and Insulin Secretion Before Diagnosis of Type 2 Diabetes: An Analysis From the Whitehall II Study. The Lancet. 2009;373(9682):2215-2221 — PMID: 19515410.
Daka B, Rosén T, Jansson PA, Råstam L, Larsson CA, Lindblad U. Inverse Association Between Serum Insulin and Sex Hormone-Binding Globulin in a Population Survey in Sweden. Endocrine Connections. 2012;2(1):18-22 — PMID: 23781314.
McLaughlin T, Abbasi F, Cheal K, Chu J, Lamendola C, Reaven G. Use of Metabolic Markers to Identify Overweight Individuals Who Are Insulin Resistant. Annals of Internal Medicine. 2003;139(10):802-809 — PMID: 14623617.
Álvarez-Villalobos NA et al. Acanthosis Nigricans in Middle-Age Adults: A Highly Prevalent and Specific Clinical Sign of Insulin Resistance. International Journal of Clinical Practice. 2020 — PMID: 31769902.
This content is for educational purposes only and does not constitute medical advice. Fasting insulin interpretation should be done alongside a full clinical picture, not as a standalone diagnostic test. The triglyceride-to-HDL ratio is a supportive marker with known limitations across different populations and should not be used in isolation. Any decision to pursue evaluation or treatment should be made individually with a qualified physician after a full evaluation. Individual results vary, and no outcome is guaranteed. Dr. Wilcox is licensed to practice in Texas, Arizona, and Florida.
