Did they rule out BOTH types of diabetes insipidus? There is one variant where your pituitary gland isn't producing antidiuretic hormone, but your kidneys are totally normal--this is called central diabetes insipidus. The other one, called nephrogenic DI, is when there is something actually wrong with your kidneys at the molecular level. How did they rule out DI? Did they try you on a trial of ddAVP?
Somebody mentioned glomerulonephritis (GN)....I highly doubt it's that. Frequent urination CAN happen with that condition, but more importantly, you'd be losing a lot of protein and/or blood in your urine and there would be other effects on your body (like being really bloated and edematous). Plus, GN is actually not one entity....it's an umbrella term which covers a ton of different conditions. It's kinda like saying cancer....cancer is obviously not one disease, but many different ones.
Has anyone looked in to the possibility of something like Addison's disease?