Unit Converter
Insulin
Synonyms
- Insulin
- Serum insulin
- Fasting insulin
- Immunoreactive insulin (IRI)
- Endogenous insulin
- Total insulin
Units of Measurement
- pmol/L
- µIU/mL
- mIU/L
Key Conversions
1 µIU/mL = 6 pmol/L
1 pmol/L = 0.1667 µIU/mL
1 µIU/mL = 1 mIU/L (direct unit shift)
Thus:
- mIU/L = µIU/mL × 1
- pmol/L = µIU/mL × 6
Description
Insulin is a 51-amino-acid peptide hormone produced by pancreatic β-cells in the Islets of Langerhans.
Released in response to:
- Rising blood glucose
- Amino acids
- Incretins (GLP-1, GIP)
Physiological roles:
- Facilitates glucose uptake in muscle & adipose tissue
- Suppresses hepatic glucose production
- Stimulates glycogen synthesis
- Inhibits lipolysis & ketogenesis
- Promotes protein synthesis
Measured clinically to evaluate:
- Insulin resistance
- Hyperinsulinemia
- Hypoglycemia (endogenous vs exogenous)
- PCOS
- Insulinoma
Physiological & Metabolic Role
1. Glucose Regulation
Insulin lowers blood glucose by:
- Increasing GLUT-4 translocation
- Suppressing hepatic gluconeogenesis
2. Lipid Metabolism
- Inhibits hormone-sensitive lipase → ↓ lipolysis
- Promotes fatty acid & triglyceride synthesis
3. Protein Metabolism
- Increases amino acid uptake
- Inhibits proteolysis
4. Appetite & Weight Regulation
High insulin → energy storage → obesity risk when chronic.
Clinical Significance
High Insulin (Hyperinsulinemia)
1. Insulin Resistance
Most common cause. Seen in:
- Obesity
- Prediabetes / Type 2 diabetes (early)
- PCOS
- Metabolic syndrome
Often characterized by:
- Elevated fasting insulin
- Elevated post-load insulin (OGTT)
2. Insulinoma
Pancreatic β-cell tumor → excessive endogenous insulin
Features:
- Whipple's triad
- Elevated insulin with high C-peptide
- Hypoglycemia with detectable insulin
3. Exogenous Causes
- Overdose of insulin therapy
(C-peptide low, insulin high)
4. Post-prandial Hyperinsulinemia
Seen in:
- Early T2DM
- Reactive hypoglycemia
- Post-bariatric surgery dumping syndrome
5. Drugs / Conditions
- Sulfonylureas (↑ insulin, ↑ C-peptide)
- Corticosteroids
- Acromegaly
- Cushing syndrome
Low Insulin
1. Type 1 Diabetes Mellitus
Autoimmune β-cell destruction → very low or absent insulin.
2. Late Stage Type 2 Diabetes
β-cell burnout after prolonged hyperinsulinemia.
3. Pancreatic Diseases
- Chronic pancreatitis
- Pancreatectomy
- Pancreatic cancer
4. Prolonged Fasting / Starvation
5. Severe Stress States
Catecholamines suppress insulin.
Reference Intervals
Fasting Insulin
- 3 – 20 µIU/mL
- 18 – 120 pmol/L
Interpretive notes:
- < 3 µIU/mL → consider Type 1 DM, β-cell failure
- 20 µIU/mL → suggests insulin resistance
- 50–60 µIU/mL → significant hyperinsulinemia
OGTT Stimulated Insulin (2 hours)
- Normal: < 50–60 µIU/mL
- Insulin resistance: 60–150 µIU/mL
- Severe resistance: >150 µIU/mL
Hypoglycemia Evaluation (during 72-hr fast)
- Insulin > 3 µIU/mL when glucose <55 mg/dL is INAPPROPRIATE
- Insulin > 6 µIU/mL strongly suggests insulinoma/sulfonylurea effect
Diagnostic Uses
1. Evaluate Insulin Resistance
With fasting glucose:
- HOMA-IR = (Glucose × Insulin) / 405
- 2.0 mild
- 2.5 moderate
- 3–4 severe (population dependent)
- 2.0 mild
2. Diagnose Insulinoma
High insulin with high C-peptide, low glucose.
3. Differentiate Endogenous vs Exogenous Hypoglycemia
- Endogenous → insulin HIGH, C-peptide HIGH
- Exogenous → insulin HIGH, C-peptide LOW
4. PCOS Evaluation
Often elevated fasting/post-load insulin.
5. Metabolic Syndrome
Insulin is an early marker before glucose rises.
6. Post-bariatric Surgery Hypoglycemia
Excessive post-prandial insulin spikes.
Analytical Notes
- Fasting sample (8–12 hrs) preferred
- Hemolysis minimal effect
- Insulin unstable at room temperature → quick separation needed
- High-dose biotin can interfere with assays
- Immunoassays may cross-react with insulin analogs
Clinical Pearls
- Insulin should never be interpreted alone—always correlate with C-peptide, glucose, and clinical context.
- High fasting insulin is an early signal of metabolic disease, years before glucose abnormalities.
- A normal or low insulin level during hypoglycemia rules OUT insulinoma.
- SGLT2 inhibitors cause fasting insulin to fall (due to lower glucose).
- Obesity is the strongest modifiable factor raising fasting insulin.
Interesting Fact
Insulin was first purified in 1921, and the first human insulin was used in 1922, transforming type 1 diabetes from a fatal disease into a chronic manageable condition.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Hormones & Insulin.
- ADA Standards of Care - Diabetes & insulin evaluation.
- Endocrine Society Guidelines - Hypoglycemia & Insulinoma.
- Mayo Clinic Laboratories - Insulin Assay.
- ARUP Consult - Insulin & C-peptide Interpretation.
- NIH / MedlinePlus - Insulin Test.