Speculation about breast implants was sparked when the Last Song actress stepped out in a dress with a revealing neckline at CNN Heroes: An All-Star Tribute on Sunday in Los Angeles.
Cyrus responded to a follower on her Twitter by saying: "Thank you for the compliment but these babies are all mine. I wish they'd realise you don't have to be fake to be beautiful!"
She then added: "HA! Yeah! And I'm 19!!! BOOBS GROW PEOPLE!"
Cyrus has also recently taken a stand with Kelly Osbourne against cyber bully on the microblogging site.
The two stars asked Twitter to start "removing people" who abuse and harass others on the social network.
Watch the trailer for The Last Song below: