It very simple to understand that if your dog is displaying hair loss symptoms then his skin is irritated enough to make him worry the area until the hair is removed and a bald spot appears.

Underneath the hair you might find very flaky skin.  In some canines you will find bald spots on the back near the tail. 

Usually the most common culprits are fleas and Mites.  Get a good flea shampoo, and I will warn you that practically all flea shampoos have poisons in them to kill the fleas.  Don’t let your pet ingest any of the poisons.  These poisons are sometimes the same poisons that are used in treating lawns.  So you get the idea.

If the shampoo makes your dog lethargic or not himself, stop using it.

Hormones can also cause hair loss.

Cushing’s disease can cause hair loss. This is very serious so be sure to have your vet check for this disease. It can kill your dog if not treated.