What is an OGTT (a GTT)?

This page is based on my experience and on what I’ve learned throughout the years. I am not a doctor, if you have further questions you can ask me, but I only know answers related to my particular case. Ask your doctor.

GTT or OGTT stands for (oral) glucose tolerance test. Is is the administration of glucose to determine how quickly it is cleared from the blood. In acromegaly this test is necessary to see how your growth hormone (GH) evolves after a large intake of glucose.

To prepare for the OGTT you should be fasting overnight (8 – 14 hours before the test), water is not always allowed. Smoking is not allowed. Two to three days before the test, you should keep a normal life, eat normally and not exercise too hard. What I do is just my normal life and I don’t go to the gym. On the day of the test, try not to do much. When I was at the hospital, I was only allowed to go to the toilet for (ahem) nr. 1 but was not allowet to take a shower. I had to remain in bed, lying, quiet, all the time. In normal life, that’s difficult so what I do is I take a shower the night before and I ask someone to drive me to the doctors on the morning of the test.

Usually the OGTT is scheduled to begin in the morning (07:00-08:00) and a zero time (baseline) blood sample is drawn. The patient is then given a glucose solution to drink. The standard dose is 75 g. It should be drunk within 5 minutes. It is extremely sweet. I find it disgustingly sweet. Sometimes it comes with a flavour, tea or raspberry juice, some other times, just nothing (I prefer it with a flavour). The first time I drank it, I felt sick and vomited, so I had to return another day to complete the test. Since then, it’s been ok. I feel sick for a while and then it’s over. But each person is different, many don’t feel anything. I, myself, I never liked candy, not even as a child!, so drinking 75g of sugar is a nightmare for me…

Once the solution is drunk, you just remain sitting, blood is drawn at intervals for measurement of glucose (blood sugar), and sometimes insulin levels. The intervals and number of samples vary according to the purpose of the test. For GH (growth hormone) supression, it takes about 2 hours. Sometimes even up to 3. So, take a book with you, it’s going to be boring.

This test is also used in the diagnosis of other conditions, such as gestational diabetes. In acromegaly patients it is used because GH is related to glucose. Drinking 75 to 100 g of glucose solution lowers blood GH levels to less than 1 nanogram per milliliter (1 ng/ml) in healthy people. In people with GH overproduction (acromegaly), this suppression does not occur, the GH levels may rise, remain constant or supress, but not enough. The oral glucose tolerance test is a highly reliable method for confirming a diagnosis of acromegaly.

It is needed to perform such a long test to see how the GH reacts to the glucose. GH peaks for a variety of reasons throughout the day and a single test at a given moment can show a very high value of GH that does not correspond to the normal (basal) value. Peaks (in healthy people) can even reach 45 ng/ml! That’s another reason why the OGTT lasts for a few hours and you have blood drawn a few times. Not only they want to see how your body supresses GH, but they need to be sure they are not measuring GH during a peak.

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