My hairs are extremely damaged by hot straightening iron if I shave off my head will my head grow healthy thick hairs?

Nobody knows- perhap. Nobody knows for sure. It may, perhaps. You can only try and see what happens. If there is no damage to the scalp- like scar tissue etc from the hot iron- and as long as your nutrition is good- yes, i think there is a good chance that you hair will be better than your hair now. I don't know about whether it is going to be thick hairs-but i think, it would be better than your hair now.