Q. What problems causes increased thirst in older dogs
DR. ANDREA M. BRODIE, DVM
A. Increased thirst in older dogs can be caused by diabetes mellitus, hyperadrenocorticism, renal failure, dehydration, diarrhea and other loss of fluids. These are the main culrpits. You should measure your dog's 24hr water intake (approx 60 ml/kg/2.2lbs). IF it is higher than that, then you should take your dog to a veterinarian for some blood tests to determine the cause.
DAVID ELBEZE, DVM, MRCVS
A. There are many conditions that could cause this symptom, among them - Diabetes, Kidney failure, Hypothyroidism, Cushing's disease, Severe internal infection, Brain tumor and urinary infection.
I would advice taking your dog for a urine analysis and a blood test in order to diagnose the cause.