A 45-year-old woman with an abnormal glucose tolerance test

Case scenario

A 45-year-old woman who had been feeling generally tired and unwell for several months presented for a full medical check up. She was overweight (body mass index [BMI], 29 kg/m²) but appeared to have no other obvious health problems. Recently performed blood tests, including a full blood count and biochemistry, had been normal. The woman was very concerned that she could have diabetes because she had read on the internet that sometimes people with insulin resistance have trouble with their weight. Therefore, I organised an oral glucose tolerance test (OGTT) with a 75 g glucose load, and simultaneous insulin levels.

The results were unexpected. Her normal fasting blood sugar level of 5.1 mmol/L fell to 3.6 mmol/L after the first 60 minutes and this was associated with her feeling quite unwell and tired. By the end of the second hour, the blood glucose level had risen to only 4.3 mmol/L. Her insulin levels were 7.9, 31.7 and 13.5 mU/L at baseline, one and two hours, respectively. (These levels did not indicate insulin resistance, according to the laboratory.) What is happening, and could this be related to her general feeling of fatigue and malaise?