How active is your dog? Is he the "leader" of your home or are you? The panting could be anxiety related.
We have a nearly 13-year-old shepherd mix rescue dog who was always "on guard" in the home. Within 3 months of finding and adopting her, she started displaying fear aggression and went ballistic if anyone came to the door or in the house. We did the worst thing possible by shrinking her world and locking her up when company came over, instead of working on socializing her.
I know it sounds nuts, but last year I started applying Cesar Millan's (The Dog Whisperer on Nat'l Geo Channel) techniques to both my dogs. The biggest thing is exercise and mastering The Walk with you as the Pack Leader. Yes, you have to work on yourself as well as your dog, and you have to get up and out of the house - for you and your dog.
For years this dog panted loudly and heavily in her sleep. After a month of getting her out in the world and properly exercised, the panting went away and never came back. I really believe it was all the stress and anxiety of thinking she had to be On Duty all day that screwed up her sleep. Now we're all sleeping soundly through the night. :-)
If you don't get the Nat'l Geographic Channel, Cesar has books and DVDs that I'm sure you could get at the library or a video rental store. The more recent book, "Cesar's Way" was very helpful to me.
ACTH Stim Test and abdominal ultrasound were normal. Still no diagnosis and the panting continues!
No cause for my golden's panting was found today at the referral center. We're awaiting results of a full thyroid panel being done at Michigan State University and a urine cortisol creatinine ratio.
My dog is a 6-7 year old spayed female Golden Retriever who pants nearly all the time now. She's anxious and tired from it all. She have an appointment at a referral hospital on Monday. I'll let you know what they say.
I forgot to add that my panting dog has seen a veterinary acupuncturist for the last three months and been treated with various herbs and acupuncture for a yin deficiency--all to no avail and no change in symptoms.
Unfortunately some dogs do pant more, as they get older. It sounds as if your veterinarian checked for all the usual suspects: heart disease, lung disease, heartworm disease, and Cushing