Insulin level (Insulin Assay)
Sample for Insulin
- This test is done on the serum of the patient.
- A fasting sample is preferred.
- Freeze the sample.
- With OGTT draw a fasting sample and then at 30, 60. and 120 minutes.
- Avoid hemolysis.
Purpose of the test (Indications) of Insulin
- It helps in the management of glucose metabolism.
- Insulin level helps in the diagnosis of insulinoma.
- Insulin levels can be advised in starting the initial therapy for patients with type 2 Diabetes Mellitus.
- In case of lower insulin levels, the most adequate is insulin or an insulin secretagogue as the treatment of choice.
- Insulin level is advised in case of fasting hypoglycemia.
- It helps the patients with diabetes requiring insulin as treatment and the patients who can control with diet only.
- Advised in the case of abnormal carbohydrate and lipid metabolism.
- Insulin was recently advised in case of polycystic ovary syndrome.
- Ladies with this disease have insulin resistance and abnormal carbohydrate metabolism that may respond to oral hypoglycemic agents.
- Insulin is not clinically advised for the diagnosis of diabetes mellitus.
- People were estimating insulin levels in OGTT to help early diagnose diabetes mellitus.
Precautions for insulin
- Keep in mind that insulin antibodies can interfere with radioimmunoassay.
- The patients treated with insulin develops antibodies.
- Obesity and food may increase the insulin level.
- Radioisotopes if given in before the test will affect the result.
- Drugs like corticosteroids, oral contraceptives, and levodopa increase the insulin level.
Pathophysiology of Insulin
Definition of Insulin:
- It is an islet cell, an anabolic hormone that controls glucose uptake, fat, and protein synthesis.
- It is a protein hormone produced by the beta cells of islets of Langerhans in the pancreas.
- Its molecular weight is 6000 D and consists of 51 amino acids.
- It has two amino acid chains A and B joined by the disulfide bridge.
- Insulin in humans is similar to insulin in animals.
- Now mostly the patients are treated with human recombinant insulin.
- Insulin forms from the preproinsulin which consists of 100 amino acids and is inactive. It is not found in the blood.
- Preproinsulin converts into proinsulin which cleaves by the proteolytic enzymes.
- There is the formation of insulin and C-peptide.
- C-peptide has no insulin-like action, but it can be measured to assess the β-cell function.
- Insulin is stored in secretory granules, when plasma glucose is increased, insulin is released by an exostosis.
- insulin half-life is 10 to 25 minutes (another reference is 4 to 9 minutes).
Insulin regulates carbohydrate metabolism (Insulin functions):
- Insulin maintains blood glucose at a constant level within the defined normal range.
- Insulin facilitates the movement of glucose from the bloodstream into the cell.
- Insulin is the anabolic hormone that stimulates the uptake of glucose into the fat and muscles.
- Insulin promotes the conversion of glucose into glycogen or fat storage.
- Insulin inhibits glucose production from the liver.
- Insulin stimulates protein synthesis and inhibits protein breakdown.
- The insulin level secretion is dependent upon the level of glucose.
Insulin secretion is reduced during fasting and starvation.
- Glucose decreases in the first three days of the fast at around 18 mg/dL.
- Glucagon secretion is doubled to maintain the glucose level.
- Lipolysis and liver ketogenesis is stimulated.
- Ketone bodies, fatty acids, and glycerol levels in the blood are increased.
- Initially triglyceride increases and later decreases.
- Cholesterol also decreases.
- Due to catabolism in starvation, the concentration of the protein in the blood increases.
- There are chances to develop metabolic acidosis.
- Reduced blood pH.
- Reduced pCO2.
Insulin levels can be evaluated with OGTT or fasting glucose:
- There are suggestions that insulin level on the same sample with OGTT level is more reliable than a single insulin level.
- Patient with Juvenile diabetes has a typical picture in OGTT:
- Low fasting insulin level and display flat GT insulin curve.
- This shows that there is no increase in the insulin level.
- Patients with mild diabetes mellitus show:
- Normal fasting insulin level.
- GT curve with a delayed rise in insulin.
- Patients with diabetes mellitus type 2 show:
- Fasting insulin level is normal and there is a delayed response of the insulin to glucose.
- Insulinoma has a characteristic picture of the fasting glucose level and insulin level.
- There is increased insulin with low fasting glucose.
- Persistent hypoglycemia with an increased insulin level.
- The insulin/glucose ratio should be less than 0.3.
- The insulin/fasting glucose ratio is more than 0.3 in Insulinoma.
Turner amended the ratio calculated by his formula:
- When the glucose level increases, the insulin level also increases when the glucose level decreases then the insulin secretion stops.
- Insulin levels will be low in Insulin-dependent diabetes mellitus.
- While insulin level is high in the non-insulin-dependent diabetes mellitus but due to resistance to insulin patient needs more insulin.
- This is the tumor of beta cells in the islets cells of Langerhans of the pancreas.
- There is persistent hypoglycemia and may be below 30 mg/dL.
- There is raised level of insulin.
- There is a persistently high level of C-peptide.
- The Turner (modified) insulin/glucose ratio is above 50.
Normal insulin level
- Infants and prepubertal child = <13 µU/mL
- Prepubertal child and adult = <17 µU/mL
- To convert into SI units x 6.945 = pmol/L
- Insulin immunoreative
- 2 to 12 year = <10 µU/mL
- Adult = <35 µU/mL
- Insulin with oral glucose tolerance test
- 0 minute = 3 to 28 µU/mL
- 30 minutes = 20 to 112 µU/mL
- 60 minutes = 29 to 88 µU/mL
- 120 minutes = 22 to 79 µU/mL
- 180 minutes = 4 to 62 µU/mL
- Adult = 6 to 26 µU/mL (43 to 186 pmol/L)
- Newborn = 3 to 20 µU /mL
- Possible critical value = >30 µU/mL
Increased Insulin level is seen in:
- Fasting insulin level is >50 µU/mL in the presence of normal or low blood glucose.
- Insulin autoimmune syndrome.
- There are raised glucose levels.
- overproduction of growth hormone in these patients gives rise to constant stimuli for the production of insulin.
- Cushing’s syndrome.
- There is a raised level of glucose by the overproduction of cortisol.
- There is constant stimulation for the production of insulin.
- Pancreatic islet cell hyperplasia.
- Obesity. There is a constant raised level of insulin.
- Fructose or galactose intolerance.
- Cirrhosis is where insufficient clearance from the blood circulation.
Decreased or absent Insulin level is seen in:
- Diabetes mellitus type 1 (maybe total lake or very low).
- In diabetes mellitus type 2, there is a low level of resistance to insulin.
- In the case of severe diabetes mellitus with ketosis and weight loss.
- In less severe cases of diabetes, insulin is present but at a lower level of glucose.
- Critical Value of Insulin = >30 mU/mL
Questions and answers:
Question 1: What will be the level of insulin in insulinoma.
There is hypoglycemia and raised level of insulin.
Question 2: What is the relation of blood glucose level and insulin level.
Insulin level is increased with increased level of glucose in the blood.