Hi BreBrash and welcome. From what I have learned here in the past few weeks Purina is not the best choice. Check out the dog and cat food forum. There is a lot of good information there regarding dog food. I don't know if this has anything to do with your dog's thinning coat but I think you should definitley check it out.
Prin (one of the more experienced members here) helped me with a recommendation of dog food for my little guy. I was very impressed with the ingredient list. Hopefully she'll be along soon and give you more info than I can.
You can also do a search and turn up information that way.