How to get thick hair?

When I was little, I had really thick hair but now it's kind of thin. Mostly in the back. My hair is really long and I brush it extremely carefully and I take hair and nail vitamins. Thin hair and thick hair are equally hereditary, I'm hoping it's one thing I didn't get from my dad's side (it's even the women on my dad's side). Is there a better way in addition to vitamins that I could try to fix this problem. It's not missing patches of hair, it's just thin. I like to wear ponytails (because they look cute on me, I know it's not the 90s anymore) but the line on the back of my head when i part it is wide and embarres me. Please help, I don't like having thin hair!