Glucose tolerance test is used for hypoglycemia. You have to be almost dead for mainstream doctors to consider you hypoglycemic. However you might get some useful information from it regardless of what the HMO doc says.
For example, my chiropractor was able to order it for me. My fasting level was around 88 (quite normal). However an hour after consuming the "sugar drink" my blood Sugar was almost 10 pts below that. 10 pts above fasting probably would have been normal.
This helped me to understand that I could not use Sugar to try to keep me alert and motivated during the day. While trying to do that I was on an energy rollercoaster. It also made my personality weird.
I later learned that I needed catecholamine support, including a little medication for inattentive ADD. It plus related supplements helped me to be more alert and less tempted to use sugar, stress and caffeine to maintain alertness. There is much more to my story but you get a feel for how the test might be useful.