There are a number of reasons for this presentation. A urinalysis and complete blood profile will help begin to answer the cause.
My rule-outs would be: diabetes insipidus,diabetes mellitus, psycogenic thirst that has led to medullary wash-out, liver shunt, renal dysplasia, polycystic kidney disease, nephroblastoma, infectious nephritis and the list goes on.
Do not be surprised if your veterinarian orders abdominal x-ray films, abdominal ultrasound, urine culture and various hormonal assays to further refine the diagnostic work-up.
Sorry that is not an easy fix, and the reason is that many serious problems can present just like this, but the astute owner and veterinarian will avoid quessing, thereby guaranteeing the best outcome possible.
Dr. Van Lienden
Dr. Raymond Van Lienden DVM
The Animal Clinic of Clifton
12702 Chapel Road, Clifton
Virginia, U.S.A. 20124