Synonym

N/A

Tubes

• Red or tiger top tube
• Lavender top tube also acceptable
• 5-7 mL of venous blood

Additional information

• Fasting for 10-12 hours before test
• Ensure that the patient is relaxed and recumbent for 30 mins before test
• Instruct the patient to withhold corticotropin, corticosteroids, oral contraceptives, thyroid supplements, epinephrine, and other medications for 3 days before test (when possible)
• Handle samples gently to prevent hemolysis
• Pack sample in ice and send the sample to lab immediately
• In patients with insulinoma, fasting for this test may precipitate severe hypoglycemia (have appropriate therapy available)
• Insulin levels may be performed in conjunction with glucose tolerance testing (GTT) or fasting blood glucose (FBG)

Info

• Insulin levels are performed to measure the concentration of the hormone insulin (either endogenous or exogenous) in the blood
• Insulin is a protein hormone synthesized by the beta cells of islets of Langerhans in the pancreas as a result of the proteolytic cleavage of proinsulin
• The actions of insulin include:
• An important regulator of blood glucose levels
• Stimulates glucose uptake by adipose tissue (fat) and skeletal muscles
• Promotes the formation of glycogen
• Stimulates protein synthesis and storage and inhibits protein degradation
• Stimulates synthesis and storage of triglycerides
• High blood glucose, amino acids, and certain pancreatic and gastrointestinal hormones such as glucagon, gastrin, secretin stimulate the pancreas to secrete insulin. Insulin secretion is inhibited by hypoglycemia and somatostatin

Clinical

• The clinical utility of serum insulin levels include:
• Evaluation of fasting and postprandial hypoglycemia
• Aids in the differential diagnosis of hyperinsulinemia as well as hypoglycemia as a result of:
• Insulinomas
• Tumor or hyperplasia of pancreatic islet cells
• Glucocorticoid deficiency
• Severe hepatic disease
• Aids in the differentiation of insulin-resistant diabetes and non-insulin-resistant diabetes mellitus
• Evaluate uncontrolled insulin-dependent (type 1) diabetes
• Insulinomas are neuroendocrine tumors arising chiefly from the islets of Langerhans and resulting in endogenous hyperinsulinism. About 90% of insulinomas are benign and 10% are malignant
• 5% of insulinomas are associated with multiple endocrine neoplasia type 1 (MEN 1)
• Insulinoma may clinically present as:
• Hypoglycemia (postprandial; 85%)
• Altered level of consciousness (due to hypoglycemia)
• Amnesia
• Blurred vision
• Diaphoresis
• Diplopia
• Palpitations
• Seizures (due to hypoglycemia)
• Tachycardia
• Weakness
• Weight gain (20-40%)
• Insulinoma can be characterized clinically by Whipple’s triad:
• Episodic hypoglycemia with low circulating glucose concentration of <50 mg/dL
• Central nervous system dysfunction presenting as confusion, anxiety, stupor, convulsions, coma
• Dramatic relief of symptoms on glucose administration
• Normal fasting blood sugar levels with increased insulin levels is suggestive of insulin resistance, which is the common feature of obesity, type 2 diabetes, and polycystic ovary syndrome
• Insulin-dependent diabetes (Type I diabetes) is primarily caused by inadequate production and/or secretion of insulin, thereby low levels of insulin are seen. In Type 2 Diabetes, there may be increased plasma insulin, but tissues are resistant to its action
• Increased insulin and decreased glucose may be seen in patients with falciparum malaria (8% cases), which may be due to consumption of glucose by large numbers of organisms and quinine may induce insulin secretion

Additional information

• Normal day-to-day variation is about 15-25% with lower levels during sleep and pregnancy. Insulin levels are higher in Hispanics than in whites
• Mean fasting insulin values are lower in infants and children than in adults. Healthy individuals have lower levels of insulin secretion in response to meals and have lower basal levels
• Some of the laboratory features of insulinoma include:
• Persistent hypoglycemia (glucose < 30 mg/dL) together with hyperinsulinemia (> 20 µIU/mL or > 139 pmol/L) after tolbutamide injection (rapid rise and rapid fall)
• Absence of insulin antibodies
• C-peptide levels should be elevated in insulinoma and decreased with self administration of insulin
• Increased insulin-to-glucose ratio (> 0.3)
• Increased proinsulin levels
• Increased proinsulin:insulin ratio
• Estimation of C-peptide along with insulin levels is useful in insulinoma work-up to rule out factitious hypoglycemia as C-peptide levels are low with the surreptitious injection of insulin and increased when endogenous levels of insulin are elevated
• The American Diabetes Association recommendations for the diagnosis of diabetes do not include the measurement of insulin levels
• Factors interfering with test results include:
• Failure to follow dietary restrictions
• Failure to withholding specified medications
• Agitation and stress
• Recent radioactive scans or radiation
• Improper collection tube such as heparin (false elevations in double antibody methods) or failure to pack sample in ice and send it to lab promptly
• Hemolysis due to rough handling of sample (false low values)
• Presence of both insulin autoantibodies and antibodies to therapeutic insulin (false low values in some assays or false high in others)
• Proinsulin cross-reacts with most antibodies
• Surreptitious insulin injection (insulin level will be elevated, with low C-peptide values)
• Surreptitious use of oral hypoglycemic agents
• In the second to third trimester of pregnancy, there is a relative insulin resistance
• Hemodialysis (destroys insulin)
• Drugs (given below)
• Related laboratory tests include:
• C-peptide
• C-peptide suppression
• Fructosamine
• Glucose
• Glucose tolerance tests (GTT)
• Glycated hemoglobin
• Hemoglobin A1C
• Microalbumin
• Tolbutamide tolerance
• Alpha, beta, and immunoreactive human chorionic gonadotropin (hCG)

Nl Result

Consult your laboratory for their normal ranges as these may vary somewhat from the ones listed below.

High Result

Conditions associated with elevated insulin levels include:

• Acromegaly
• Alcohol use
• Cushing’s syndrome
• Excessive administration of insulin
• Familial persistent hyperinsulinemic hypoglycemia
• Infants of diabetic mothers
• Insulinoma
• Insulin resistance syndrome (metabolic syndrome)
• Neonatal hyperinsulinemic hypoglycemia
• Obesity (most common cause)
• Pancreatic islet cell hyperplasia
• Severe liver disease
• Stress
• Obesity
• Post-prandial
• Pregnancy
• Renal failure
• Type 2 diabetes mellitus
• Uremia
• Drugs
• Oral Hypoglycemic Agents
• Acetohexamide
• Adenosine
• Albuterol
• Amino acids
• Aspirin
• Beclomethasone
• Calcium gluconate
• Cannabis
• Captopril
• Chlorpropamide
• Chlorthalidone
• Deferoxamine
• Glimepiride
• Glipizide
• Glyburide
• Human growth hormone
• Interferon alfa-2a
• Isoproterenol
• Levodopa
• Lisinopril
• Medroxyprogesterone
• Megestrol
• Methylprednisolone
• Metoprolol
• Niacin
• Nicotinic acid
• Norethindrone
• Oral contraceptives
• Perindopril
• Prazosin
• Prednisolone
• Prednisone
• Quinine
• Rifampin
• Ritodrine
• Secretin
• Spironolactone
• Streptozocin
• Terbutaline
• Tolazamide
• Tolbutamide
• Trichlormethiazide
• Verapamil

Low Result

Conditions associated with decreased insulin levels include:

• Beta cell failure
• Ethanol
• Exercise
• Fasting
• Hypopituitarism
• Post Pancreatectomy
• Type 1 diabetes mellitus
• Drugs
• Acarbose
• Calcitonin
• Cimetidine
• Clofibrate
• Conjugated estrogen
• Diazoxide
• Diltiazem
• Doxazosin
• Enalapril
• Enprostil
• Ethacrynic acid
• Ether
• Furosemide
• Metformin
• Midazolam
• Morphine
• Nifedipine
• Octreotide
• Phenytoin
• Propranolol
• Psyllium
• Troglitazone

References

• ARUP Consult®. Insulinoma. [Homepage on the Internet]©2007. Last reviewed in June 2007. Last accessed on November 20, 2007. Available at URL: http://www.arupconsult.com/Topics/OncologicDz/NeuroendocrineTumors/Insulinoma.html
• ARUP's Laboratories®. Insulin, Free & Total. [Homepage on the Internet]©2007. Last accessed on November 20, 2007. Available at URL: http://www.aruplab.com/guides/ug/tests/0070155.jsp
• eMedicine from WebMD®. Insulinoma. [Homepage on the Internet] ©1996-2007. Last updated on July 25, 2006. Last accessed on November 20, 2007. Available at URL: http://www.emedicine.com/med/topic2677.htm
• Kappert K et al. Insulin facilitates monocyte migration: A possible link to tissue inflammation in insulin-resistance. Biochem Biophys Res Commun. 2007 Nov 10; [Epub ahead of print].
• Kotronen A et al. Tissue specificity of insulin resistance in humans: fat in the liver rather than muscle is associated with features of the metabolic syndrome. Diabetologia. 2007 Nov 16; [Epub ahead of print].
• Laboratory Corporation of America. Insulin. [Homepage on the internet]©2007. Last accessed on November 20, 2007. Available at URL: http://www.labcorp.com/datasets/labcorp/html/chapter/mono/sr002600.htm
• LabTestsOnline®. Insulin. [Homepage on the Internet]© 2001-2007. Last reviewed on December 20, 2006. Last accessed on November 20, 2007. Available at URL: http://www.labtestsonline.org/understanding/analytes/insulin/glance.html
• Van Pelt RE et al. Insulin Secretion and Clearance After Subacute Estradiol Administration in Postmenopausal Women. J Clin Endocrinol Metab. 2007 Nov 6; [Epub ahead of print].