I'm sorry to hear that your boy is going through this. You mention that you've tried, some medications and shampoos as well as having his thyroid checked, correct? In addition to this, there are a few other things to try. First, consider having a RAST (blood test) done. This can show food allergies and may help you to alter his diet (a common cause). There's a low allergy food called Hill's Z/D that you can talk with your vet about. Next, if it hasn't been done yet, get a skin scraping. These can show if mites, infections etc., are present. Adding 1 T of plain yogurt to his food daily may help a bit as can an over the counter product called Derma Pet. Some dogs with thyroid problems also have adnreal issues -or- may have immune system problems (mentione mast cells to your vet); have this looked into via bloodwork as well. Finally, try changing some environmental factors. Some common culprits are: floor cleaners (try using vinegar and water instead), cedar chips, certain types of grasses, and even some flooring (laminates etc). This should help to pinpoint his issue. I hope he's doing better soon.
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