A. that is a little surprising as it sounds like it could be renal failure. I would probably advise doing a urine analysis as well to the check the specific gravity of the urine, this case can sometimes detect early renal failure that do not show on blood test.
Other conditions could cause excessive drinking as well, such as hypothyroidism, urine infection, intestinal infection/blockage, Cushing's disease and others.
Most of the diseases mentioned will not always show any abnormal values in a normal blood test, they might require other specific blood tests or a ultrasound.
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