Question from Portland, Oregon, USA:

I am a 31 year old female that was recently diagnosed with diabetes. I am 5 feet, 4 inches and weigh 128 pounds, so, I am not overweight. I was diagnosed with gestational diabetes during pregnancy almost two years ago, but testing a year after the baby was born showed my post meal levels were still elevated (180 to 250 mg/dl [10.0 to 13.9 mmol/L]).

My doctor initially diagnosed me with type 2, but a blood test showed low C-Peptide and insulin levels that were below normal. Diet and exercise did very little to bring down my levels, unless I ate virtually no carbohydrates at all. He sent me to a endocrinologist, suspecting type 1. Further testing showed I am GAD antibody negative. He put me on a fast acting insulin that I take before meals, which is working. My fasting levels are usually between 100 and 140 mg/dl [5.6 and 7.8 mmol/L] before taking the insulin.

My mother, age 57, who is quite thin and active, was also recently diagnosed with type 2. Her levels are very similar to mine, raising very quickly after meals to the 200 to 250 mg/dl [11.1 to 13.9 mmol/L] range and she is not responding well to diet and exercise. Yet, her blood sugars are not elevated enough to seem like type 1. She has not yet had any blood tests. Her mother and grandfather, on mother’s side, both had supposed type 2, but also were thin. My younger sister, age 26, recently tested herself after a meal and was at 200 mg/dl [11.1 mmol/L], so it looks like she could be diabetic as well, although she is overweight. My dad also has type 2 and is thin, but his is related to hemochromatosis. He was diagnosed at age 55. I have been tested for this and do not have it.

I am confused as to what kind of diabetes this is. My endocrinologist is very interested in my case, but doesn’t seem to know exactly what form this is, other than it seems linked to genetics. He doesn’t seem to think it’s type 2 because of the C-peptide and insulin level tests, my age and body type. Could it be MODY, since there appears to be such a strong genetic factor? Which type of diabetes is this likely to be?