Get checked ASAP. First test will look for diabetes mellitus, in blood and urine, as this is how very high sugars look. May also have diabetes insipidus, another hormone imbalance. A chemistry panel and urinalysis are definitely good starting points. I'd at least go to urgent care for this, if not the er, unless you get same day appointment with your doc. Can have very bad electrolyte imbalances.
See a doctor. Insatiable thirst even after drinking copious amounts of water generally means there is a chemical imbalance in the blood. Most commonly, this occurs in people who are diabetic and do not yet know they have the disease. Often when a person is first diagnosed, they will report constant thirst as well as fatigue, malaise, change in appetite, and frequent urination, all due to high blood sugar.