You need to take her to the vet to be tested for diabetes or thyroid conditions. Increased water intake is one of the symptoms of these two conditions. Also bring a stool sample. You vet will test her blood sugar level and do blood test. This will identify if she has either one of these conditions, which are both treatable. If she does not have either, the blood test will help to determine if it is kidney or liver problem or an illness like Cushings disease.
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Peace and happiness to you and your pet! ~ Christina
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