Insulin

Insulin is an anabolic hormone that promotes glucose uptake, glycogenesis, lipogenesis, and protein synthesis of skeletal muscle and fat tissue through the tyrosine kinase receptor pathway. In addition, insulin is the most important factor in the regulation of plasma glucose homeostasis, as it counteracts glucagon and other catabolic hormones—epinephrine, glucocorticoid, and growth hormone.

Table 1. Reference Range of Insulin Levels ^[1] (Open Table in a new window)

A standard insulin test is positive for endogenous insulin and exogenous insulin. In addition, there is a minimal cross-reaction with proinsulin and insulinlike growth factors 1 and 2, with the degree of variability depending on the brand of the testing toolkit and technique used.

Insulin testing is used to assist in identifying causes of hypoglycemia (plasma glucose levels < 55 mg/dL), especially upon signs and symptoms of hypoglycemia (neurohypoglycopenic and autonomic symptoms). In this scenario, a 72-hour fasting test is performed. ^[2]

• Insulinoma: High insulin and C-peptide levels

• Non–beta cell tumors: Low insulin and C-peptide levels and high insulinlike growth factor 2 level ^[3]

• Excessive insulin administration: High insulin levels and low C-peptide levels

• Insulin secretagogue administration (sulfonylurea and glinides): High insulin and C-peptide levels

• Congenital hyperinsulinism (mutation in insulin-secreting gene): High insulin and C-peptide levels

• Autoimmunity to insulin or insulin receptor (common in patients receiving insulin or those who have autoimmune diseases such as systemic lupus erythematosus [SLE] or Hashimoto thyroiditis): Postprandial insulin is bound to antibodies and dissociated 1 hour later, resulting in an extremely elevated insulin level and high insulin–to–C-peptide ratio ^[4]

Table 2. Interpretation of 72-hour Fasting Test Results ^[2] (Open Table in a new window)

Conditions associated with elevated insulin levels

Conditions associated with increased insulin resistance ^{[4, 5]} (beta cell compensates via hypersecretion of insulin) include the following:

• Obesity

• Steroid administration

• Acromegaly

• Cushing syndrome

• Insulin receptor mutation ^[4]

• Type 2 diabetes (early stage)

Research suggests that mitochondrial reactive oxygen species are linked to the development of insulin resistance in adipose tissue and skeletal muscle. ^[6]

A study by Gabay et al indicated that insulin resistance in female adolescents with obesity may have a negative impact on brain structure and function. The report found, for example, that in the study population, the insular cortices in such patients tended to be thinner, a phenomenon that was not seen in male adolescents with obesity and insulin resistance. ^[7]

Conditions associated with increased insulin secretion include the following:

• Insulinoma (insulin or proinsulin secreting tumors)

• Administration of insulin secretagogues

Excessive administration of insulin is associated with elevated insulin levels.

Conditions associated with decreased insulin excretion include the following ^[4] :

• Severe liver disease

• Severe heart failure (liver congestion)

Autoimmunity to insulin or insulin receptor is associated with elevated insulin levels.

Conditions associated with decreased insulin levels

Conditions associated with beta-cell destruction include the following:

• Post pancreatectomy

• Chronic pancreatitis

• Autoimmune destruction

• Type 1 diabetes

In type 2 diabetes (late stage), beta cells fail to secrete insulin for maintaining the blood glucose level, owing to insulin resistance and genetic defect. ^[8]

Increased insulinlike growth factor levels are associated with non–beta cell tumors.

Method: Radioimmunoassay; enzyme-linked immunosorbent assay (ELISA)

Specifics for collection and panels are as follows:

• Specimen type: Blood serum

• Container: Vacutainer, red top

• Collection method: Venipuncture

• Specimen volume: 1 mL

• Measure blood glucose and C-peptide level in same sample

• An 8-hour fasting specimen required

Other instructions

A 72-hour fasting test is used to identify causes of postabsorptive hypoglycemia. ^[2] The patient is instructed to fast, and plasma glucose, insulin, proinsulin, and C-peptide levels are measured every 6 hours until the plasma glucose level is less than 65 mg/dL, after which the testing frequency is increased to every 1-2 hours. Fasting is ended when plasma glucose levels are less than 45 mg/dL accompanied by signs and symptoms of hypoglycemia. At the endpoint, a blood sample is collected and tested for glucose, insulin, proinsulin, C-peptide, beta-hydroxybutyrate, and sulfonylurea levels. The patient is given 1 mg of intravenous glucagon, and the response of the blood glucose level is measured.

Description

Biosynthesis ^{[9, 4, 10]}

Insulin is a peptide hormone that is secreted from beta cells of the islets of Langerhans in the pancreas. It is initially synthesized in endoplasmic reticulum and Golgi apparatus as proinsulin; it is then cleaved to insulin and C-peptide. Although insulin and C-peptide are cosecreted in equal molar proportions, the ratio of serum insulin to C-peptide is 1:5-15. Fifty to sixty percent of insulin is extracted by the liver before it reaches systemic circulation, and it has a half-life of only 4 minutes. In contrast, C-peptide and proinsulin are excreted via the kidney. ^[8]

Function

Insulin is an anabolic hormone that promotes glucose uptake, glycogenesis, lipogenesis, and protein synthesis of skeletal muscle and fat tissue through the tyrosine kinase receptor pathway. In addition, insulin is the most important factor in the regulation of plasma glucose homeostasis, as it counteracts glucagon and other catabolic hormones—epinephrine, glucocorticoid, and growth hormone.

Secretion

In normal physiology, insulin secretion is induced by elevated plasma glucose levels. Glucose diffuses to beta cells through glucose transporter 2 (GLUT2) and activates the glycolysis pathway, leading to elevated adenosine triphosphate (ATP) levels. Increasing ATP levels induce ATP-sensitive K⁺ channels to shut down and subsequently stimulate depolarization of the beta-cell membrane. Then, voltage-gate Ca²⁺ channels are opened to increase cytosolic Ca²⁺ and trigger insulin exocytosis. ^[8] However, high insulin levels in a hypoglycemic state have been found in a hypersecretory state; an example is insulinoma, in which insulin is secreted in at a high rate independent from the plasma glucose level.

Interestingly, oral administration of glucose is more effective in increasing insulin secretion than intravenous glucose (called "incretin effect"). Carbohydrate meals potentiate insulin secretion through multiple gastrointestinal hormones (incretin hormones), including cholecystokinin, glucagonlike peptide-1 (GLP-1), and gastric-inhibiting polypeptide (GIP). ^{[8, 11]}

Indications/Applications

Insulin testing is used to assist in identifying causes of hypoglycemia (plasma glucose levels < 55 mg/dL), especially upon signs and symptoms of hypoglycemia (neurohypoglycopenic and autonomic symptoms). In this scenario, a 72-hour fasting test is performed. ^[2]

Insulin levels can also be used to assess insulin resistance/sensitivity. In addition, insulin testing is used to help differentiate type 1 from type 2 diabetes.

Considerations

Insulin levels may be falsely elevated by the following:

• Amino acid (leucine, arginine, and lysine)

• Steroid

• Insulin secretagogue (sulfonylurea and glinide)

• Estrogen ^[12]

• Beta2 agonist

Insulin levels may be falsely decreased by the following:

• Acarbose, metformin, octreotide, and beta-blocker

• Hemolysis (insulin-degrading enzyme in red blood cell released) ^[4]

• Hemodialysis

Insulin levels can also be used to assess insulin resistance versus sensitivity. In insulin resistance, the ability of cells to respond to the action of insulin in transporting glucose into tissues is diminished; consequently, the resistant individual begins secreting above-normal amounts of insulin to obtain a quantitatively normal response.

Insulin resistance develops long before the appearance of disease signs. A study by Kraft found borderline diabetes in 14% of subjects with normal oral glucose tolerance tests who had been randomly referred for such evaluation. ^[13]

There are multiple methods available to assess insulin resistance, including the following:

• Hyperinsulinemic-euglycemic glucose clamp
• Fasting insulin
• Homeostasis model assessment (HOMA)
• Quantitative insulin sensitivity check index (QUICKI)
• Kraft insulin patterns/protocol
• Hayashi protocol

Each of these methods has its own limitations. The lack of standardization of the insulin assay procedures prevents the comparison of results between studies; consequently, studies can be compared only qualitatively. The American Diabetes Association (ADA) organized a task force to standardize insulin assays. ^[14]

HOMA equations have been one of the tools widely used in research to estimate insulin resistance. The two equations (which use fasting blood levels) are as follows, with HOMA-IR used to assess insulin resistance and HOMA-B used to assess pancreatic beta cell function ^{[15, 16]} :

• HOMA-IR = (glucose in mmol/L x insulin in mIU/mL)/22.5
• HOMA-B = (20 x insulin in mIU/mL)/(glucose in mmol/L - 3.5)

Fasting insulin levels can serve as a tool to help guide the choice of therapy in patients newly diagnosed with type 2 diabetes. A study by Saxena et al found that such patients with normal to low initial fasting serum insulin levels responded better to glipizide than to metformin. On the other hand, those with high fasting serum insulin levels responded significantly better to metformin than to glipizide. ^[17]

Using the HOMA index, a pediatric study by Genovesi et al indicated that, just as a relationship between degree of insulin resistance and risk of hypertension exists in children with excess weight, the same holds true in youngsters of normal weight. The investigators noted that while children with higher body mass index (BMI) z-scores had greater HOMA index values, the HOMA index was linked to the systolic blood pressure z-score regardless of weight class. ^[18]