Is your tiel a Lutino?
Its often common for a lutino to have a bald patch right behidn thier crest. My bird has a very very tiny one you cant see unless she has her crest all the way up and you blow on the top of her head, lol. Dont ask!
Anyway, the bald spots can be on other mutations besides Lutinos, its just prevalent with them. From what i had read I belive it was cuase by the imbreeding used to create the lutino mutation. Could be wrong on that.
Bald spots are nothing horrible. Birds with bald spots still make great pets, its just they shouldnt be used as breeders to prevent the passing of the bald gene.
Now if your bird had a full head of feathers and is al of a sudden loosing feathers from the head. Thats a different story.
In that case. How old is your bird? When was its first molt or next molt due?