http://www.eastbournevet.co.uk/pethe...p=T&animal=dog
Increased thirst is often noted by owners, especially if there is only one dog in the household. If there are more dogs present, this may not be noted for some time. Often the dog will also urinate more frequently, or in larger quantities, and this can always be noted by an attentive owner. There are many different reasons for dogs to drink, and urinate, more than normal. It is generally not thought to be a problem if the dog drinks less than 100 ml per kilogram body weight per 24 hours. This would be as much as 3 litres (about 6 pints) for a 30kg (65lb) Labrador! If your dog drinks more than this amount, there is nearly certainly something wrong! It has to be remembered that the water intake of a dog is dependent on many factors, such as the weather, the type of food (dry or tinned), the activity level of the dog or the physical status of the dog (pregnant, lactating etc.).
Increased thirst is nearly always related to increased urination and some of the conditions mentioned below are primarily conditions causing increased urination and secondarily increased drinking.
One of the more common reasons for dogs to drink excessively is kidney disease, especially in the older dog.
Liver disease is another common reason. Diabetes Mellitus is also possible in dogs. Furthermore, there are several hormonal conditions which can cause excessive thirst, such as Cushing’s Disease, Diabetes Insipidus, Hyperthyroidism, Hyperparathyroidism etc. Another common condition, especially in older female dogs, is pyometra: this is an infection of the uterus. There are some more, less common, conditions as well, causing excessive thirst.
In most cases, your vet will have to do several tests, mostly blood tests, to find out what the reason for the excessive thirst is in your dog. It is always advisable to visit your vet if your dog drinks excessively.