Overview
Insulin is a peptide hormone produced by the beta cells of the pancreatic islets and is the principal anabolic hormone in the human body. It plays a central role in regulating the metabolism of carbohydrates, fats, and proteins. Insulin is essential for maintaining normal blood glucose levels and overall metabolic balance.
One of the primary actions of insulin is to control glucose production and release by the liver while promoting glucose uptake and utilization by peripheral tissues. Insulin levels and insulin catabolism show an inverse relationship, meaning reduced insulin action leads to increased glucose levels. Insulin works in opposition to glucagon, another pancreatic hormone, and together they maintain glucose homeostasis.
Physiological Role of Insulin
Insulin facilitates the uptake of glucose by insulin-sensitive cells such as skeletal muscle and adipose tissue. Once inside the cells, glucose is either utilized for energy production or stored for future use.
In the liver and skeletal muscles, insulin promotes the conversion of glucose into glycogen, a storage form of glucose. This action lowers blood glucose levels after meals. Insulin also suppresses gluconeogenesis, thereby reducing the production of glucose by the liver.
In fat metabolism, insulin inhibits fat breakdown and promotes fat storage. It encourages the conversion of excess glucose into fatty acids and triglycerides. In protein metabolism, insulin stimulates protein synthesis and inhibits protein breakdown, supporting tissue growth and repair.
Insulin, Glucagon, and Glucose Relationship
Insulin and glucagon have opposing effects on blood glucose levels. When blood glucose levels rise after food intake, insulin secretion increases. This promotes glucose uptake by tissues and glycogen formation, resulting in a reduction of blood glucose levels.
When blood glucose levels fall, insulin secretion decreases and glucagon release increases. Glucagon stimulates glycogen breakdown in the liver, releasing glucose into the bloodstream. This coordinated interaction ensures stable glucose levels under fasting and fed conditions.
Indications for Insulin Testing
Insulin estimation is indicated in individuals presenting with symptoms of hypoglycemia such as excessive sweating, hunger, confusion, dizziness, blurred vision, and fainting. It is also useful in patients with heart disease, obesity, and polycystic ovary syndrome.
Insulin testing plays an important role in the evaluation of diabetes mellitus, insulin resistance, metabolic syndrome, and hypoglycemia. It is used to monitor insulin therapy and to assess pancreatic islet cell function, including after islet cell transplantation. The test also helps clinicians decide on insulin therapy initiation in patients with type 2 diabetes.
Sample Collection and Patient Preparation
A minimum of 8 hours of fasting is required before sample collection. Blood samples are usually collected twice for evaluation.
The first sample is a fasting sample, and the second sample is collected 2 hours after a normal meal. Patients are advised to consume their usual full diet before submitting the post-prandial sample. Insulin, glucose, and C-peptide tests are often performed together for comprehensive assessment.
Estimation Methods
Insulin is commonly measured using sandwich electrochemiluminescence immunoassay (ECLIA). This method uses biotinylated and ruthenium-labeled monoclonal antibodies to form antigen–antibody complexes.
Magnetic microparticles capture these complexes, and chemiluminescent emission is measured quantitatively. Other methods include enzyme-linked immunosorbent assay (ELISA) and radioimmunoassay, though the latter is less commonly used.
Normal Reference Ranges
Normal insulin reference ranges vary depending on the assay method. Fasting insulin levels are typically less than 25 mIU/L.
Post-prandial insulin levels range from 16 to 166 mIU/L. For conversion to SI units, insulin values in mIU/mL can be multiplied by 6.0 to obtain pmol/L.
Insulin Measurement During GTT
Insulin estimation may also be performed as part of glucose tolerance testing. After fasting sample collection, glucose is administered orally at a dose of 1.25 grams per kilogram of body weight.
Subsequent blood samples are collected at 30, 60, and 120 minutes. Expected insulin values rise after glucose intake and gradually decline as glucose is utilized. This method helps evaluate insulin secretion patterns and insulin resistance.
Interpretation of High Insulin Levels
Elevated insulin levels indicate hyperinsulinemia. This condition is commonly associated with type 2 diabetes, obesity, insulin resistance, metabolic syndrome, and polycystic ovary syndrome.
High insulin levels are also linked to cardiovascular disease and non-alcoholic fatty liver disease. Markedly elevated insulin levels may indicate insulinoma, a rare insulin-secreting tumor of the pancreas.
Interpretation of Low Insulin Levels
Low insulin levels are seen in type 1 diabetes due to beta-cell destruction and insulin deficiency. They may also indicate advanced beta-cell failure in long-standing type 2 diabetes.
Low insulin levels can be observed following exogenous insulin administration and in conditions affecting pancreatic function.
Combined Interpretation of Insulin and Glucose
Simultaneous interpretation of insulin and glucose levels provides better diagnostic insight. High insulin with low glucose suggests hyperinsulinemic hypoglycemia or insulinoma.
High insulin with high glucose indicates insulin resistance or type 2 diabetes. Low insulin with high glucose is typical of type 1 diabetes, while low insulin with low glucose may be seen in severe liver disease, adrenal disorders, or certain medications.
Clinical Significance
Insulin testing is a valuable tool in diagnosing and managing diabetes, insulin resistance, and hypoglycemic disorders. It supports treatment decisions and helps monitor metabolic health when interpreted alongside glucose and C-peptide levels.
