“Glucose Insulin Tolerance Test” is not the usual name for a standard test today. It may refer to an older combined test that tracked blood sugar after insulin and glucose, or be confused with an insulin tolerance test (ITT), which deliberately lowers blood sugar to check hormone responses. A glucose tolerance test is different: it checks how the body handles a glucose drink and is commonly used to assess diabetes.
What Is a Glucose Insulin Tolerance Test?
The term glucose-insulin tolerance test has been used in historical medical literature to describe a combined test involving glucose and insulin. The aim was to study how the body regulated blood sugar and responded to changes in glucose levels. A historical paper in The Journal of Clinical Endocrinology & Metabolism discussed the combined glucose-insulin test alongside the glucose tolerance test and insulin tolerance test as separate methods of investigating disorders of glucose metabolism.
Today, the exact test name matters because these procedures answer different medical questions. A glucose tolerance test is primarily used to examine how the body processes sugar, while an insulin tolerance test is a specialist endocrine investigation that measures how the body responds to insulin-induced low blood sugar. They are not interchangeable, and neither should be assumed to be the test intended when someone uses the combined term.
How Does It Differ From a Glucose Tolerance Test and an Insulin Tolerance Test?
Although their names are similar, the tests use different procedures and measure different responses. The following comparison can help you understand which investigation may have been requested.
| Test | What happens | Main purpose |
|---|---|---|
| Glucose tolerance test (GTT or OGTT) | You drink a measured glucose solution, and blood samples are taken before and after it. | Assess how the body processes glucose and help diagnose diabetes or gestational diabetes when the appropriate protocol is used. |
| Insulin tolerance test (ITT) | Insulin is administered under medical supervision to lower blood glucose temporarily, followed by repeated blood sampling. | Assess the body's growth hormone and cortisol responses, particularly when pituitary or adrenal hormone function is being investigated. |
| Combined glucose-insulin tolerance test | Historical protocols combined insulin and glucose administration with measurements of the blood sugar response. | Investigate blood sugar regulation and related endocrine responses in selected clinical or research settings. |
According to the US National Library of Medicine's MedlinePlus guidance on glucose tolerance testing, the oral glucose tolerance test is commonly used to assess how the body handles a glucose drink. Intravenous glucose tolerance testing is different and is rarely used in routine care. This distinction is important because a test involving glucose and insulin measurements does not automatically have the same purpose as either a standard glucose tolerance test or a specialist insulin tolerance test.
Why Might a Doctor Request One of These Tests?
The reason depends on the exact procedure ordered. A glucose tolerance test may be requested when a clinician needs more information about how the body processes sugar. An insulin tolerance test may be considered when there is a clinical need to assess the production of growth hormone or the body's regulation of cortisol. A historical combined glucose-insulin test should not be assumed to be a routine modern investigation for diabetes.
Checking how the body handles glucose
During a glucose tolerance test, blood sugar is measured before and after a standard amount of glucose is consumed. The results show how blood glucose changes over a defined period. This test may be used to investigate diabetes or to assess for gestational diabetes during pregnancy, following the relevant pregnancy-specific protocol.
The MedlinePlus guide to diabetes tests explains that clinicians can use different blood tests to screen for, monitor, or diagnose diabetes. The appropriate test depends on the clinical situation, including whether pregnancy is involved.
Assessing pituitary and adrenal hormone function
An insulin tolerance test works differently. Insulin lowers blood glucose, and this temporary drop normally triggers responses involving growth hormone and cortisol. Growth hormone is released by the pituitary gland and plays a role in growth and body composition. Cortisol is produced by the adrenal glands and helps the body respond to physical stress, illness, and other demands.
Cambridge University Hospitals explains that an insulin tolerance test measures cortisol and/or growth hormone after stimulation. The clinician considers the hormone levels alongside the blood glucose response to determine what the test shows about endocrine function.
How Is the Test Performed?
The exact procedure depends on the investigation named on the appointment letter. Before attending, confirm whether the appointment is for a glucose tolerance test, an insulin tolerance test, or another combined procedure. Do not try to reproduce any of these tests at home, because the protocols and safety requirements differ.
What happens during a glucose tolerance test?
For a typical oral glucose tolerance test, you arrive after fasting for the period specified by the testing service. A healthcare professional takes an initial blood sample, then asks you to drink a measured glucose solution. Additional blood samples are collected at specified times to measure how your blood glucose changes after the drink.
The test may take two to three hours, depending on the protocol. The glucose drink is very sweet, and some people experience nausea, sweating, or light-headedness. Tell the staff if you begin to feel unwell during the procedure.
What happens during an insulin tolerance test?
An insulin tolerance test is performed in a supervised clinical setting. The team may first check your weight, blood pressure, heart rhythm, and baseline blood glucose. A small tube called a cannula is inserted into a vein so that insulin can be given and repeated blood samples can be collected without inserting a new needle for every sample.
After the initial samples are taken, a measured dose of insulin is administered. The clinical team checks your blood glucose and collects further samples to measure the relevant hormones, usually cortisol and/or growth hormone. Staff monitor your response throughout the procedure and are prepared to treat low blood glucose if needed.
The University Hospitals Sussex patient guide to the insulin stress test describes this specialist procedure and the need for clinical supervision. Depending on the protocol, the sampling period lasts around two hours, and the entire visit may take longer because of preparation and recovery.
How Should You Prepare?
Follow the instructions supplied by the clinic or laboratory, because preparation requirements vary between tests. Depending on the procedure, you may need to fast overnight or for a specified number of hours. Some tests allow plain water during fasting, while food and other drinks are restricted until the sampling period is complete.
It is important to tell the healthcare team about all medicines and supplements you take, especially insulin, diabetes medicines, steroid medicines, and hormone replacement treatments. Some medicines can affect the test or change how safely it can be performed. Do not stop, delay, or change a prescribed medicine unless the clinician responsible for your test has given you specific instructions.
If you are having an insulin tolerance test, tell the team in advance if you have previously experienced seizures, have epilepsy, have angina or other significant heart problems, or may be pregnant. These circumstances may affect whether the test is suitable or whether a different investigation should be considered. Children and adults may also follow different protocols, and age-based eligibility rules can vary between specialist centres.
What Are the Risks and Possible Side Effects?
The possible side effects depend on which test you have. With a glucose tolerance test, some people experience nausea, sweating, light-headedness, or faintness after drinking the glucose solution or having blood taken. Temporary soreness or bruising may occur where a needle is inserted.
An insulin tolerance test deliberately lowers blood glucose, so it carries a greater risk of hypoglycaemia, which means blood sugar that is too low. Symptoms may include:
- Shaking or trembling
- Sweating or feeling unusually warm
- Hunger, dizziness, or light-headedness
- Weakness, drowsiness, or difficulty concentrating
- Palpitations or a feeling that the heart is beating quickly
Rarely, a substantial fall in blood glucose can cause loss of consciousness or a seizure. The test must therefore be performed by an experienced clinical team with appropriate monitoring and treatment available. If you feel unwell at any point, tell the staff immediately rather than waiting for the next scheduled blood sample.
How Are the Results Interpreted?
There is no single set of normal values that applies to every procedure called a glucose-insulin tolerance test. Results depend on the type of test, the timing of blood samples, the laboratory method, and the reason the investigation was performed.
Glucose tolerance test results
For an oral glucose tolerance test, clinicians interpret blood glucose values at specific time points using established criteria. The relevant thresholds can differ according to whether the test is being used to assess diabetes outside pregnancy or gestational diabetes. A result should be interpreted using the correct protocol rather than by applying a cutoff from a different test.
Insulin tolerance test results
During an insulin tolerance test, the clinician considers whether the blood glucose level fell sufficiently to provide a valid stimulus and whether the expected hormone responses occurred. Growth hormone and cortisol results are evaluated using the laboratory's methods and the clinical purpose of the investigation. If the intended blood glucose response was not achieved, the test may not provide a reliable answer and further assessment may be needed.
An abnormal result does not, by itself, establish the cause of a hormone problem. The clinician interprets it alongside the person's medical history, symptoms, medicines, other hormone measurements, and any further investigations that may be appropriate.
What Happens After the Test?
After a glucose tolerance test, the staff will tell you when you can eat and drink normally. If you have an insulin tolerance test, you will be monitored until your blood glucose has recovered and you are considered well enough to leave. You may be given a drink or meal after the procedure, so bringing food to eat afterward can be helpful when advised by the clinic.
You may feel tired after an insulin tolerance test. Ask the clinic whether you should arrange for someone else to take you home, and follow its instructions about driving, exercise, eating, and restarting medicines. Results may be discussed at a later appointment because hormone measurements need to be reviewed together rather than in isolation.
Which Test Should You Ask About?
If your appointment or referral uses the phrase “glucose insulin tolerance test,” ask the referring clinician or testing unit for the exact name and purpose of the procedure. Confirm whether it involves drinking glucose, receiving insulin, measuring hormone levels, or a combination of these steps. That clarification will help you follow the correct preparation instructions and understand what the results are intended to show.