Serum insulin

Reference values (norm) for insulin concentration in blood serum in adults are 3-17 μU/ml (21.5-122 pmol/l).

Insulin is a polypeptide, the monomeric form of which consists of two chains: A (of 21 amino acids) and B (of 30 amino acids). Insulin is formed as a product of proteolytic cleavage of the insulin precursor, called proinsulin. Insulin itself is formed after leaving the cell. The cleavage of the C-chain (C-peptide) from proinsulin occurs at the level of the cytoplasmic membrane, which contains the corresponding proteases. Insulin is necessary for cells to transport glucose, potassium and amino acids into the cytoplasm. It has an inhibitory effect on glycogenolysis and gluconeogenesis. In adipose tissue, insulin enhances glucose transport and intensifies glycolysis, increases the rate of synthesis of fatty acids and their esterification and inhibits lipolysis. With prolonged action, insulin increases the synthesis of enzymes and DNA, activates growth.

In the blood, insulin reduces the concentration of glucose and fatty acids, as well as (albeit slightly) amino acids. Insulin is relatively quickly destroyed in the liver by the enzyme glutathione insulin transhydrogenase. The half-life of insulin administered intravenously is 5-10 minutes.

The cause of diabetes mellitus is insulin deficiency (absolute or relative). Determining the concentration of insulin in the blood is necessary to differentiate various forms of diabetes mellitus, select a therapeutic drug, select optimal therapy, and determine the degree of β-cell deficiency. In healthy people, when performing OGTT, the concentration of insulin in the blood reaches a maximum 1 hour after taking glucose and decreases after 2 hours.

Impaired glucose tolerance is characterized by a slower rise in blood insulin concentration relative to the increase in glycemia during OGTT. The maximum rise in insulin levels in these patients is observed 1.5-2 hours after glucose intake. The content of proinsulin, C-peptide, and glucagon in the blood is within normal limits.

Type 1 diabetes mellitus. The basal concentration of insulin in the blood is within normal limits or decreased, a smaller increase is observed at all times of the OGTT. The content of proinsulin and C-peptide is decreased, the level of glucagon is either within normal limits or slightly increased.

Type 2 diabetes mellitus. In the mild form, the fasting blood insulin concentration is slightly elevated. During the OGTT, it also exceeds normal values at all times of the study. The content of proinsulin, C-peptide and glucagon in the blood is unchanged. In the moderate form, an increase in the fasting blood insulin concentration is detected. During the OGTT, the maximum insulin release is observed at the 60th minute, after which there is a very slow decrease in its concentration in the blood, so high insulin content is observed 60, 120 and even 180 minutes after the glucose load. The content of proinsulin, C-peptide in the blood is reduced, glucagon is increased.

Hyperinsulinism. Insulinoma is a tumor (adenoma) consisting of β-cells of the pancreatic islets. The tumor can develop in people of any age, it is usually single, benign, but can be multiple, combined with adenotosis, and in rare cases - malignant. In the organic form of hyperinsulinism (insulinoma or nesidioblastoma), sudden and inadequate insulin production is observed, which causes the development of hypoglycemia, usually paroxysmal. Insulin hyperproduction does not depend on glycemia (usually above 144 pmol / l). The insulin / glucose ratio is more than 1: 4.5. Excess proinsulin and C-peptide are often detected against the background of hypoglycemia. The diagnosis is certain if, against the background of hypoglycemia (blood glucose concentration less than 1.7 mmol/l), the plasma insulin level is higher than 72 pmol/l. Tolbutamide or leucine loads are used as diagnostic tests: patients with an insulin-producing tumor often show a high increase in blood insulin concentration and a more noticeable decrease in glucose levels compared to healthy individuals. However, the normal nature of these tests does not exclude a tumor diagnosis.

Many types of malignant tumors ( carcinomas, especially hepatocellular, sarcomas) lead to the development of hypoglycemia. Most often, hypoglycemia accompanies tumors of mesodermal origin, resembling fibrosarcomas and localized mainly in the retroperitoneal space.

Functional hyperinsulinism often develops in various diseases with carbohydrate metabolism disorders. It is characterized by hypoglycemia, which can occur against the background of unchanged or even increased insulin concentrations in the blood, and increased sensitivity to the administered insulin. Tests with tolbutamide and leucine are negative.

Diseases and conditions in which insulin concentrations in the blood change

Insulin is elevated

• Normal pregnancy
• Type 2 diabetes mellitus (onset of the disease)
• Obesity
• Liver diseases
• Acromegaly
• Itsenko-Cushing syndrome
• Muscular dystrophy
• Insulinoma
• Familial fructose-galactose intolerance

Insulin is low

• Long-term physical activity
• Diabetes mellitus type 1
• Diabetes mellitus type 2

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