I agree with BSC about the thyroid. That is a common one that causes fatigue. Make sure your doctor runs both Free T4 and Free T3 because you can have a normal TSH with the other 2 being abnormal. Your Free T3 is your energy and that level should test around the top 2/3rds of the lab range. If your BS is running high, then that could cause fatigue but if you are doing fairly good, then that shouldn't cause the fatigue.
One time when I was in college, I was so tired I literally slept for 24 hours straight!