dog hair loss

my 10 year old long haired chihuahua is losing her hair-checked thyroid and it was o.k. now trying change in dog food from store brand to purina--nothing that indicates a problem that started this hairloss

I have no idea what's wrong with your dog, but Purina is a very cheap low quality corn-based food, and can be responsible for allergies in dogs.

I would try feeding her a premium food (Wellness, Innova, California Natural, Nutro) and see if that helps the problem.