I have the same problem with my 8 year old male flat coat. After numerous testing with all results negative he was treated with methylprednisone. Didn't seem to help. Now 2 months later, it is apparent that his whole coat has thinned out and although he has the bald streak down the top of his tail, his whole tail has thinned out drastically.

During a regular vet check-up I talked to the dr about the thinning on the tail. He tested my dog (female) for thyroid with a simple blood test and found out that her levels were way beyond normal and diagnosed her as being 'hypo-thyroid'. She has just started taking medication.