3. Other hormonal diseases (Estrogen excess, estrogen deficiency, growth hormone-responsive alopecia, ) If your dog has a tumor that is causing extra estrogen, his or her skin and coat will start turning dark about the belly, and then the hair will become brittle and fall out. She will need testing to determine if this is the problem, but can be treated by being spayed or neutered. If there is not enough estrogen, the hair falls out on the belly. Hormonal diseases can only be diagnosed after blood tests, so you need to take her to your vet.