Impaired glucose tolerance: A transition phase between normal glucose tolerance and diabetes, also referred to as prediabetes. In impaired glucose tolerance (IGT), the levels of blood glucose are between normal and diabetic. People with IGT do not have diabetes. Each year, only 1-5% of people whose test results show IGT actually develop diabetes. And with retesting, as many as half of the people with IGT have normal oral glucose tolerance test results. Weight loss and exercise may help people with IGT return their glucose levels to normal. With diet and exercise as many as 60% will not developed diabetes or it will be considerably slowed in its development.