Why have I always had a thin patch of hair?

I noticed looking back at photos from when I was about 5 up until my current age (18) that I have always had a patch of hair that is very thin whilst the rest is thick. It's not thinning it's just always been there but only in a certain patch! It is causing me a great deal of stress and often you can see my scalp when styling! Anyone know why this is? And how to fix it maybe? I've considered fue hair transplant! Thanks!