Hi. The very high ALT would suggest liver and bile duct disease as the most obvious. Although it can also be high with any ingestion of poisons, pancreatitis and Cushings, diabetes, hyperthyroidism, heartworm treatments, and hemolytic anemia. The BUN/creatinine ratio is only very slightly out of range. The alkaline phosphatase is very high and again points to liver issues. Cushings is also a potential issue, as is heart disease. High Triglycerides tend to point to liver issues again, but also potentially hypothyroidism. High platelet counts are quite rare. The most likely is a connection to liver issues or Cushings.
I think therefore your vet is going down the right route. Discounting Cushings is certainly the first priority, followed by looking more closely at liver issues and having a thyroid function (T3/T4) test undertaken.
Tony