I just saw your other responses, I only had the first because I was writing back when you were writing!
Where he is older, I would still keep heart on the list, even if he is not coughing when he wakes up.
Depending on when the thyroid testing was done, I would also keep that on the list. Basic bloodwork usually gives clues to look more into Cushing's though...I do always say that compared to a human life span, going to the vet every 3-4 months when you're 16 or 17 is like us going to the doctor's once a year when we're 70.
If the tests are truly all normal, and there is nothing wrong with his heart, I would go after the cognitive dysfunction, or plain old dreaming more. If he shows any signs of confusion while awake, or changes in personality, ask your vet if he is okay for the medication for this.
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