Excessive thirst is a sign of several different types of problems that only upon examination and testing can a doctor diagnose.
The illness most associated with excessive thirst that comes most quickly to mind is diabetes; which is a serious and even life threatening disease if left untreated. I'm not saying this is what you have it is just what comes to mind first.
If you have had a change in lifestyle such as exercising more; it is possible you are not replacing the fluids you are loosing through exercise. Being dehydrated is serious and is something you need to be educated on how to prevent.
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