Endocrinology & Diabetology & Internal Medicine & Family Medicine
Insuline à jeun | Clinique Omicron Québec
Fasting insulin is the measurement of serum insulin concentration after a standardized fast of at least 8 hours, reflecting basal secretion by pancreatic beta cells in the absence of dietary stimulation. Unlike postprandial insulin or insulin measured during a glucose load, fasting insulin provides specific information on the level of autonomous insulin secretion required to regulate basal blood glucose levels and suppress hepatic endogenous glucose production. Its main clinical use in primary care is to assess insulin resistance by calculating the HOMA-IR (Homeostasis Model Assessment - Insulin Resistance) index, which combines fasting insulin and fasting blood glucose in a validated mathematical model. A high fasting insulin result - with normal or slightly elevated fasting blood glucose - indicates compensatory hyperinsulinism characteristic of insulin resistance, present in metabolic syndrome, polycystic ovary syndrome (PCOS) and pre-diabetic states. Conversely, low fasting insulin is observed in type 1 diabetes (autoimmune destruction of beta cells) and in some advanced forms of type 2 diabetes (beta cell exhaustion). It's essential to remember that fasting insulin has major limitations: high inter-individual and intra-individual variability, interference by anti-insulin antibodies (in patients already on exogenous insulin), and lack of universal standardization between dosing methods - which is why HOMA-IR is best interpreted with the standards of the laboratory and reference study used.
Physiology, reference values, and calculation of HOMA-IR
- Basal insulin secretion and the significance of fasting insulin: under fasting conditions (8h minimum - ideally 10-12h): basal blood glucose is maintained within the normal range (3.9-5.5 mmol/L) → by basal insulin secretion → which inhibits hepatic glucose production (glycogenolysis + gluconeogenesis) → and maintains the balance of lipolysis → basal insulin secretion represents 40-50 % of total 24h insulin secretion (the rest = prandial response) → in insulin-resistant conditions: target cells (liver + muscle + adipose tissue) respond less well to insulin → beta cells compensate by increasing their insulin secretion → compensatory hyperinsulinism → blood glucose maintained within normal or slightly elevated limits → as resistance worsens → beta cells become exhausted → fasting insulin decreases → fasting blood glucose increases → T2DM → the «bell-shape» curve of insulinemia: in the progression to T2DM: fasting insulin initially high (compensated resistance) → then gradually decreases (beta-cell depletion) → final phase: low insulin + frank hyperglycemia = T2DM established → this dynamic explains why fasting insulin should not be interpreted alone, but always with simultaneous blood glucose and HbA1c; fasting insulin reference values: normal values vary significantly according to assay methods (immunoenzymatic vs. immunochimioluminescent) and kits used → individual laboratory standards must be used → generally accepted guideline values: normal fasting insulin: 2-25 µIU/mL (14-174 pmol/L) depending on the laboratory → values >25 µIU/mL in an individual with normal blood glucose → suggestive of compensatory hyperinsulinism (insulin resistance) → values <2 µuiml en présence d'hyperglycémie → évocateur d'insuffisance des cellules bêta (dt1 ou dt2 avancé) les valeurs très élevées (>(100–1,000 µUI/mL) in the presence of hypoglycemia → suggestive of insulinoma or exogenous insulin injection (differentiate by C-peptide); HOMA-IR (Homeostasis Model Assessment — Insulin Resistance): formula validated by Matthews 1985 — Diabetologia → HOMA-IR = fasting insulin (µUI/mL) × fasting blood glucose (mmol/L) / 22.5 → or if blood glucose is in mg/dL: HOMA-IR = fasting insulin (µUI/mL) × fasting blood glucose (mg/dL) / 405 → interpretation: HOMA-IR <1.0 → normal insulin sensitivity (excellent) → HOMA-IR 1.0–1.9 → borderline → HOMA-IR ≥2.0 → mild insulin resistance → HOMA-IR ≥2.5 → moderate resistance → HOMA-IR ≥5.0 → severe resistance → thresholds vary depending on the populations studied (ethnicities + age + BMI) → some studies use a threshold of 2.0 for Caucasians + 1.8 for Asians → HOMA-IR correlates with the hyperinsulinemic euglycemic clamp (De Fronzo 1979 — American Journal of Physiology) — reference method (gold standard) but invasive and reserved for research → clinical example: patient with fasting blood glucose = 5.5 mmol/L + fasting insulin = 18 µIU/mL → HOMA-IR = 18 × 5.5 / 22.5 = 4.4 → moderate to severe insulin resistance → recommendation: lifestyle treatment (diet + exercise + weight loss)
- Prerequisites for Fasting Insulin Testing and Limitations: ESSENTIAL prerequisites for reliable dosing: strict fasting ≥8h (preferably 10-12h) → do not perform in a patient who has recently taken exogenous insulin → some drugs influence fasting insulin: drugs that increase fasting insulin: sulfonylureas (glibenclamide + gliclazide) → directly stimulate beta-cell secretion → artificially elevated insulinemia → meglitinides (repaglinide + nateglinide) → same effects → anabolic steroids + androgens → increase IGF-1 → potentiate insulin signaling → corticoids: increase insulin resistance → fasting insulin may be elevated + drugs decreasing fasting insulin: metformin (reduces hepatic glucose production → decreases insulin requirement → slightly reduced fasting insulin) → GLP-1 agonists + SGLT2i → reduce basal insulin secretion by reducing blood glucose + analytical interferences : endogenous anti-insulin antibodies (in patients treated with old bovine or porcine insulin - very rare) + autoimmune anti-insulin antibodies (insulin autoimmune syndrome - rare - Hirata) → very high false results + intra-individual variability: fasting insulin may vary by 30-50 % from one day to the next in the same patient → a single assay may be insufficient → average 2 to 3 measurements for better accuracy → in a research context → HOMA-IR calculated on the average of 3 consecutive fasting samples + limitations of HOMA-IR: only assesses liver insulin resistance (hepatic glucose production) → does not reflect muscle resistance (euglycemic clamp assesses both) → not recommended as a universal screening tool in routine clinical practice → but useful in research + epidemiology + individualized follow-up of patients with metabolic syndrome or PCOS
Clinical interpretation based on results
| Result / context | Interpretation and course of action | References and recommendations |
|---|---|---|
| High fasting insulin + normal blood sugar — insulin resistance Metabolic syndrome — PCOS — obesity — prediabetes — acanthosis nigricans — HOMA-IR — lifestyle treatment — metformin — SGLT2i — GLP-1 |
Elevated fasting insulin (>15-25 µIU/mL) + normal fasting blood glucose (3.9-5.5 mmol/L) → compensatory hyperinsulinism → insulin resistance → HOMA-IR ≥2.5 → course of action: confirm with fasting blood glucose + HbA1c + lipid panel (triglycerides + HDL + LDL) + waist circumference → metabolic syndrome assessment (IDF 2005: ≥3 criteria out of 5: high TT + TG ≥1.7 mmol/L + low HDL + fasting blood glucose ≥5.6 mmol/L + BP ≥130/85 mmHg) → if PCOS suspected (woman of childbearing age + oligo-anovulation + hyperandrogenism): FSH + LH + testosterone dosage + ovarian ultrasound → HGPO 75g if prediabetes suspected → clinical signs of insulin resistance: acanthosis nigricans (verrucous hyperpigmentation nape + armpits + groin - sign of severe hyperinsulinism) + abdominal obesity (TT ≥ 94 cm male / ≥80 cm female) + acrochordons (molluscum pendulum - small skin growths in the folds) + treatment: weight loss (5-10 % of weight): reduction of insulin resistance by 30-50 % → regular physical exercise (150 min/week + muscle resistance): activation of AMPK pathway → increase in GLUT4 → improvement in muscle sensitivity + diet: reduction in refined carbohydrates + saturated fats → Mediterranean or DASH diet → metformin: reduction in hepatic glucose production + improvement in peripheral sensitivity → indicated if prediabetes + high-risk factors (Diabetes Canada 2023 + ADA 2024) + SGLT2i + GLP-1 agonists (semaglutide): growing evidence in insulin resistance + PCOS → weight reduction + improved insulin sensitivity + monitoring: fasting blood glucose + HbA1c every 6-12 months if documented resistance → screening for T2DM | Matthews 1985 — Diabetologia: HOMA-IR → validation + interpretation → international reference + De Fronzo 1979 — American Journal of Physiology: euglycemic clamp → gold standard insulin resistance → + Yildiz 2003 — NEJM: PCOS + insulin resistance + hyperandrogenism → mechanisms + IDF 2005: metabolic syndrome → criteria + prevalence + ADA 2024 Standards of Diabetes Care: insulin resistance + prediabetes + metformin + Diabetes Canada 2023: guidelines → insulin resistance + prediabetes + metformin + INESSS Québec: T2D prevention programs + screening + Estruch 2013 — NEJM (PREDIMED): Mediterranean diet + metabolic syndrome → reduction in MACE by 30 % |
| Low fasting insulin + hyperglycemia — beta-cell insufficiency T1DM - advanced T2DM - LADA - insulinopenia - autoantibodies - C-peptide - HbA1c - insulin therapy - diabetes classification |
Low fasting insulin (7.0 mmol/L) → insufficient beta cell secretion → insulinopenia → diagnostic orientation: T1DM: autoimmune destruction of beta cells → anti-GAD65 + anti-IA-2 + anti-ZnT8 + anti-insulin positive → C-peptide undetectable or very low (<0.2 ng/mL) → often abrupt onset + ketosis or ketoacidosis → treatment: insulin therapy mandatory + advanced T2DM (beta-cell exhaustion after years of resistance and chronic hyperglycemia): autoantibodies negative + C-peptide still present but low (0.2-0.5 ng/mL) + progressive evolution over years + treatment: if residual C-peptide → basal insulin in addition to oral antidiabetic drugs → if C-peptide almost nil → basal-bolus regimen + LADA (Latent Autoimmune Diabetes in Adults): diabetes with initial T2DM appearance (no ketoacidosis) but positive autoantibodies (anti-GAD65 in particular) + progressively low fasting insulin + declining C-peptide → initially treated as T2DM then requires insulin → Tuomi 1993 - Diabetologia: description of LADA → clinical context: fasting insulin alone is not sufficient to distinguish diabetes types → always combine with: stimulated C-peptide + autoantibodies + HbA1c + clinical context + MODY (monogenic deficiency): normal or slightly low fasting insulin depending on mutated gene → negative autoantibodies + slightly or moderately elevated blood glucose + multigenerational family history → genotyping required | Tuomi 1993 — Diabetologia: LADA → description and criteria + Greenbaum 2021 — NEJM: teplizumab + T1D → C-peptide preservation → ADA 2024 + ISPAD 2022: diabetes classification + autoantibodies + C-peptide + Diabetes Canada 2023: classification + diabetes treatment → LADA + T1D + T2D + MODY + INESSS + RAMQ Quebec: insulin therapy coverage + analogs by diabetes type |
| Very high fasting insulin + hypoglycemia - insulinoma and hyperinsulinism Insulinoma — fasting hypoglycemia — elevated C-peptide — 72h fast — insulin/glucose ratio — MEN-1 — factitious hypoglycemia — sulfonylureas — differential diagnosis |
Very high fasting insulin (>18 µIU/mL according to Endocrine Society 2009) + blood glucose 3 µIU/mL + C-peptide >0.2 nmol/L + pro-insulin >5 pmol/L → endogenous hyperinsulinism → insulinoma (90 % benign) → differential: sulfonylurea or meglitinide → elevated C-peptide but sulfonylurea detected in urine → exogenous insulin injection (sham hypoglycemia) → elevated insulin + LOW or undetectable C-peptide (as exogenous insulin does not contain C-peptide) → localization of insulinoma: abdominal MRI + pancreatic echo-endoscopy (70-90 % sensitivity) + Ga-68 DOTATATE PET → treatment: surgical resection → cure in 90 % of benign → diazoxide pre-operatively or if inoperable (inhibits beta-cell secretion by opening K⁺-ATP channels) → octreotide if SSTR2/SSTR5 receptors present → see also Insulin / C-Peptide sheet for full algorithm + Turner's insulin/glucose ratio: fasting insulin (µIU/mL) / fasting glucose (mg/dL) × 100 → if >30 → suspect endogenous hyperinsulinism → old method but still cited | Cryer 2009 — Journal of Clinical Endocrinology and Metabolism (Endocrine Society Guidelines on Hypoglycemia): diagnostic thresholds for endogenous hyperinsulinism during hypoglycemia → absolute reference + Service 1999 — NEJM: insulinoma → diagnosis + algorithm + Grant 2020 — JCEM: 72-hour fast + updated criteria + Kaplan 1986 — NEJM: intra-arterial calcium → insulinoma localization + Endocrine Society 2014 + AACE: insulinoma guidelines + hyperinsulinism + Diabetes Canada 2023 + ADA 2024: hypoglycemia in non-diabetics → diagnostic algorithm + INESSS Québec: access to diagnostic tests (insulin level + C-peptide + 72-hour fast in specialized setting) |
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Fasting insulin must ALWAYS be interpreted with the simultaneous fasting blood glucose — never in isolation: A high fasting insulin with normal blood glucose indicates insulin resistance (calculate HOMA-IR). A low fasting insulin with hyperglycemia indicates beta-cell insufficiency. A very high fasting insulin with hypoglycemia necessitates ruling out an insulinoma or surreptitious sulfonylurea use (differentiated from factitious causes by C-peptide, which is high in case of insulinoma and undetectable in case of exogenous insulin injection).
Situations requiring urgent medical attention
Very high fasting insulin (>50 µUI/mL) + documented hypoglycemia (<3.0 mmol/L) + high C-peptide + outside of any sulfonylurea or insulin treatment → endogenous hyperinsulinism (probable insulinoma) → urgent endocrinology consultation → 72-hour fast if hypoglycemia is not reproducible → abdominal MRI + pancreatic echoendoscopy → resection surgery → diazoxide while awaiting.
High fasting insulin + normal or slightly elevated blood glucose + young woman with oligomenorrhea + hyperandrogenism + HOMA-IR >3.0 Severe insulin resistance in the context of PCOS → endocrinology + gynecology consultation → combined treatment: weight loss + exercise + metformin + or semaglutide + combined oral contraceptive if hyperandrogenism → fertility assessment if desire for pregnancy.
Hypoglycemia + very high insulin + undetectable C-peptide + patient on therapeutic insulin — or suspicion of surreptitious insulin use in a non-diabetic patient → Factitious hypoglycemia due to exogenous insulin injection → Urgent psychiatric and medical consultation → Screen for sulfonylurea and exogenous insulin (anti-insulin antibodies) → No insulinoma if undetectable C-peptide.
Consult at Clinique Omicron
Clinique Omicron physicians prescribe and interpret fasting insulin as part of the workup for metabolic syndrome, insulin resistance, PCOS, and prediabetes assessment. They calculate HOMA-IR, initiate lifestyle modifications and metformin if indicated, and refer to endocrinology for organic hypoglycemia and pathological hyperinsulinism. Consultations are available at several service points in Quebec and via telemedicine. To book an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not substitute for the advice of a physician or endocrinologist. Fasting insulin should always be interpreted within the full clinical context, with simultaneous blood glucose, C-peptide, and patient medications.
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