A relatively common side effect with unregulated diabetes is drinking a lot and urinating frequently. This is because a diabetic has high level of sugar in the blood. This is filtered in the glomerulus, but the level is so high that the proximal tubule is not able to re-absorb all of the sugar. Thus, it follows relatively much sugar with the pre-urine in the nephrons. Sugar will here, as elsewhere, retain water because solutes attract water (see focus on osmosis). This increases the urine output and the patient loses fluid and become thirsty.
Much sugar is filtered over a long time in the glomerulus can be physically harmful to the kidneys due to the sugar molecule size. This is part of the reason why kidney failure are is a known side effect of diabetes.