My Dog Was Recently Diagnosed With Diabetes. We Initially Took

My dog was recently diagnosed with diabetes. We initially took him in because he was not eating and we could hear him having trouble breathing. At that time there were also discussion about his thirst (input/output). I agreed to a panel to test his organ functionality and received a diagnosis of diabetes - at this time his sugar was just over 500. He finished his antibiotic and started insulin and three weeks in his sugars are even higher - over 600. How long should I continue down the insulin road if Cushings is another possibility? Thanks, Kathy