Insulin is a peptide hormone secreted in the body by beta cells of islets of Langerhans of the pancreas and regulates blood glucose levels. Medical treatment with insulin is indicated when there is inadequate production or increased demands of insulin in the body.[1]

The FDA approved indications for treatment with insulin are as follows.

Outpatient indications for insulin:

Insulin is primarily used for the treatment of diabetes mellitus. According to the American Diabetes Association, in 2018, 34.2 million Americans or 10.5% of the U.S. population had diabetes.[2] Approximately 90 to 95% of patients with diabetes have type 2 or insulin-resistant diabetes, 5 to 10% have type 1 diabetes, and 1 to 2% have MODY.[2] 

• In Type 1 or juvenile diabetes mellitus, pancreatic beta cells are destroyed by the body’s immune system or trauma or injury to the pancreas, leading to decreased or absent insulin production.[3] Patients with type-1 diabetes always require insulin.[4][5]

• Type 2 diabetes mellitus is the most common type of diabetes and usually occurs after the age of 45 years, although children and teens are also developing it more frequently, mainly due to childhood obesity. In type 2 diabetes, cells are resistant to the action of insulin. The exact cause of this is unknown, but factors such as genetic, obesity, metabolic syndrome, or sedentary lifestyle play a role. As the pancreas secretes more insulin to try to get glucose into the cells, they eventually burn out, and glucose starts building up in the blood. Treatment with insulin is necessary for the later stages of type 2 diabetes.[6][7]

• Maturity onset diabetes of young (MODY) is an inherited condition in which beta cells of the pancreas cannot secrete insulin due to genetic mutation.[8] Patients are younger, healthy weight, without antibodies for islet cells of the pancreas and have first degree relatives with a similar type of diabetes. Insulin or sulfonylureas are treatment options.

• Gestational diabetes mellitus (GDM): Approximately 4% of all pregnancies in the U.S. are complicated by GDM.[2] Screening for GDM is performed during 24 to 28 weeks of pregnancy, and initial treatment is with diet and exercise. Treatment is necessary to prevent complications of preeclampsia, macrosomia, and shoulder dystocia. When diet and exercise fail to control the blood sugar level, insulin is indicated.[9]

Inpatient indications for insulin:

• Management of diabetes mellitus patients in the hospital: Glycemic control in patients with type 1 and type 2 diabetes admitted to general wards for conditions other than diabetes is achievable with subcutaneous or intravenous insulin infusion. This therapeutic approach is to avoid hypoglycemia, hyperglycemia, volume depletion, or electrolyte abnormalities during hospitalization.[10]

• Diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemia (HHS) are complications of type 1 and type 2 diabetes, respectively, and both represent extremes in hyperglycemia. Insulin is indicated in both DKA and HHS to normalize blood glucose levels.[11][12]

• Stress-related hyperglycemia: Hyperglycemia in non-diabetic critically ill patients, like ICU, surgical or trauma patients, is managed by insulin. In stress, hormones like catecholamine, glucagon, growth hormones, cortisol as well as glycogenolysis and gluconeogenesis, cause hyperglycemia which is related to poor outcomes in hospitalized patients. Therefore, insulin is used to maintain a blood glucose concentration of 140 to 180 mg/dl in these critically ill patients.[13]

• Treatment of hyperkalemia: IV insulin is used in the management of hyperkalemia in hospitalized ESRD patients.[14][15]

Insulin acts by directly binding to its receptors on plasma membranes of the cells. These receptors are present on all the cells, but their density depends on the type of cells, with the maximum density being on the hepatic cells and adipocytes.

The insulin receptor is a heterotetrameric glycoprotein consisting of two subunits, the alpha and the beta subunits. The extracellular alpha subunits have insulin binding sites. The beta subunits, which are transmembranous, have tyrosine kinase activity.[16] When insulin binds to the alpha subunits, it activates the tyrosine kinase activity in the beta subunit, which causes the translocation of glucose transporters from the cytoplasm to the surface of the cell.[17] These glucose transporters allow the influx of glucose from the blood into the cell, thus reducing the blood glucose levels.[18]

Insulin causes the following effects in the cells[19]:

Hepatic Cells: Promotes glycogenesis, inhibits gluconeogenesis.

Adipocytes: Promotes lipogenesis, inhibits lipolysis

Muscle cells: Promotes glycogenesis and protein synthesis. Inhibits protein catabolism

Pancreatic beta cells: Inhibits glucagon release.

Brain Cells: Involved in appetite regulation

Depending on the duration of action, insulin categorizes as short-acting, intermediate-acting, and long-acting.

Rapid-acting insulins (lispro and aspart) start their action in 5 to 15 minutes and peaks in 30 minutes. The duration of action is 3 to 5 hours. They are generally used before meals and always used along with short-acting or long-acting insulins to control sugar levels throughout the day.

Short-acting (regular insulin) starts the action in 30 to 40 minutes and peaks in 90 to 120 minutes. The duration of action is 6 to 8 hours. Patients take these agents before meals, and food is necessary within 30 minutes after its administration to avoid hypoglycemia.

Intermediate-acting insulins (NPH) starts the action in 1 to 4 hours, and peaks in 4 to 8 hours. Dosing is usually twice a day and helps maintain the blood sugar levels throughout the day.

Long-acting insulins, such as glargine and detemir, start action in 1 to 2 hours. They provide a plateau effect over 12 to 24 hours. Dosing is usually during the night time after meals. Their long duration of action helps in reducing the frequency of dosing throughout the day.

Insulin administration can be via subcutaneous, intravenous, and intramuscular routes. The route of administration usually depends on the patient condition and setting.

The subcutaneous route is the most widespread route of administration and preferred by most patients due to its ease and convenience in administration. Patients use the subcutaneous route in the form of insulin syringes, pens, and pumps. It is an easy and convenient way for patients to self administer.

Intravenous insulin is used in the hospital setting, especially when immediate and close monitoring of blood glucose levels is needed.[20] It is used in patients with diabetic ketoacidosis, hyperosmolar hyperglycemic state, severe hyperkalemia, beta-blocker toxicity, and calcium channel blocker toxicity. All of these cases require emergency treatment, and hence intravenous insulin is used.

Intramuscular insulin use is rare and utilizes concentrated regular insulin.

In 2014, the FDA approved an inhalable insulin formulation. It passes through the lungs and into the bloodstream and provides a rapid onset of action within 12 minutes. It can be taken by patients with diabetes type 1 and type 2 before meals. 

The insulin pump is a device that works like a natural pancreas. It replaces the need for long-acting insulin and continuously delivers small amounts of short-acting insulin in the body throughout the day. 

Adverse effects of insulin classify according to those caused by the drug itself and those caused by the specific route of administration.

Hypoglycemia is, by far, the most common adverse effect of insulin therapy.[21] The other adverse effects of insulin therapy include weight gain, and rarely electrolyte disturbances like hypokalemia, especially when used along with other drugs causing hypokalemia.

The subcutaneous route of administration also has its adverse effects. Pain at the injection site, lipodystrophy at the site of injection are the most common adverse effects of daily subcutaneous injections.[22] Other adverse effects like peripheral hyperinsulinemia and decreased compliance are also seen in the population using the subcutaneous route for administration of insulin.

Somogyi effect: Some patients who take insulin before bed, wake up with high blood sugar levels. This effect occurs when the insulin causes a hypoglycemic condition in the body, which activates the antihyperglycemic hormones such as cortisol and adrenaline, resulting in rebound hyperglycemia.; this can be corrected by reducing the dose of bedtime insulin or changing the time of insulin dosing.

Dawn phenomenon: It is the presence of high blood glucose levels in the body in the early hours of the day due to inadequate insulin in the body. To correct this phenomenon, the dose of bedtime insulin needs to increase to be able to keep the blood glucose levels under control throughout the night and the early hours of the morning.[23]

Though there are no absolute contraindications to insulin therapy, the dose of insulin needs to be adjusted and monitored in numerous settings.

• Insulin dosing requires adjustment in patients with renal impairment and liver failure, as insulin metabolism occurs in the liver, and it is excreted in the urine.
• Insulin dose and blood glucose levels should have careful monitoring in patients with a history of hypoglycemic episodes.
• Insulin should not be given with other drugs, which also cause hypokalemia, like diuretics.
• Vomiting and diarrhea cause a hypokalemic state in the body, and clinicians and patients should exercise care when administering insulin as it also causes hypokalemia.
• Insulin is also contraindicated in patients with cresol sensitivity as insulin preparations contain cresol.