A. The causes of excessive drinking and excessive urination include metabolic diseases, such as diabetes and Cushing's disease, kidney disease, liver disease, replacement of losses due to dehydration (usually through vomiting and/or diarrhea, but also due to loss of fluids inside the body, like effusion into the abdomen) , and some types of cancer. It's important to first figure out whether he's drinking a lot to account for the excessive urination (primary polyuria) or he's urinating a lot to get rid of the fluids from the excessive drinking (primary polydipsia). This requires blood testing and possibly imaging (x-rays, blood work) along with a thorough history and physical examination. Your dog needs to be seen by the vet as soon as possible, as these conditions are all progressive and serious.
A. If he fills the diapers and they are wet, it could explain the excessive drinking - he might have diarrhea which causes dehydration so he is always thirsty.
Diarrhea also often causes malabsorption of food and this could explain why he is hungry all the time.
The cause of the diarrhea must be investigated, take him to the vet in order to check him over and run some tests - hopefully they will be able to treat the problem.
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