Golden Retriever has mange?

I own a 10 year old male Golden and he was treated for Demodectic mange last year. The vet said it was hereditary and that he could have gotten it from his mother. Anyway he was shaved and went through a series of dips. That didn't help. Then the vet ran more test and say that he has a thyroid problem. He has been on thyroid medicine now for about 2 months and he has had one dose of Promeris flea treatment (he said it was good for loss of hair) Anyway, it hasn't grown back, in fact he looks worse. Is there anyone out there that has had a Golden Retriever with this problem. I can go back to the vet but any information you can give me would be appreciated. Thanks