My name is***** and I am sorry to hear that your dog has chronic diarrhea.
Chronic diarrhea in dogs can be caused by many different things with many different treatments. I will list the causes and treatments below and you can take this information to your vet to help you both decide on the next step of the treatment plan.
Firstly, I would recommend a complete blood profile including urine and thyroid test. I would also recommend a fecal exam to look for intestinal parasites such as hookworms and giardia. This is a good baseline to start with. The next test I would recommend would be a "GI profile." This is a group of blood tests that look at intestinal function as well as pancreas function and inflammation. The last test that could be useful is an abdominal ultrasound. This will look at the architecture and shape of the intestines and pancreas to lead to a diagnosis.
The most likely cause of chronic diarrhea is a food allergy. Unfortunately, there is no good blood test for this problem, but a diet change to a hydrolyzed protein diet will show results in a matter of days. There are several varieties of hydrolyzed protein diets made by Royal Canin, Purina and Science Diet. You can also try a limited ingredient diet such as a fish and potato based diet. The most common food allergies in dogs are beef, wheat and dairy, but we also see allergies to chicken, lamb and rice.
The next most common cause of chronic diarrhea is inflammatory bowel disease. This is often related to a food allergy, but can also have a random inflammatory component to it. The GI profile will likely lead you in the direction of IBD. Treatment for this is also a hydrolyzed protein diet as well as steroids to reduce inflammation in the intestines.
Another less common cause would be pancreas problems. The pancreas can either be inflamed or not functioning normally. The GI profile will tell you about pancreas inflammation and function.
If there is no improvement with the food trial and potentially steroids, an abdominal ultrasound could be very helpful to examine the intestines and look for a tumor or other problem in the intestines.
Does this help? What other questions do you have?