Model Gigi Hadid is battling Hashimoto's disease.

The catwalk star may be one of the biggest names in fashion, but away from work she also has to live with a secret health condition. Hashimoto's disease, also known as Hashimoto's thyroiditis, is a condition in which your immune system attacks your thyroid, a small gland at the base of your neck below your Adam's apple.

"My metabolism actually changed like crazy this year. I have Hashimoto's disease. It's a thyroid disease," Gigi told Elle.

"It's now been two years since taking the medication for it, so for the (Victoria's Secret) show I didn't want to lose any more weight. I just want to have muscles in the right place, and if my butt can get a little perkier, then that's good."

Much has been made about Gigi's shrinking frame since she found runway fame, as well as her model sister's, Bella Hadid. Bella has Lyme disease, which her brother Anwar and mother Yolanda also suffer with.

Gigi, 21, doesn't let her health problems get her down though, and as well as storming the Victoria's Secret runway for the second year in a row earlier this month (Dec16), she was also crowned Model of the Year at the recent Fashion Awards in London.

Keeping fit is a big part of Gigi's life, and she's even found away to work exercise into her down time.

"I put a sticky note in my kitchen that just said 'squats' and every time I walked past the squat sticky note I had to do 15 squats," she shared. "It's a good trick because you have to have integrity with yourself. No one else is watching but you're like, 'Okay, I have to do this for myself,' which is important."

Zoe Saldana, Gina Rodriguez and Victoria Justice also have Hashimoto's disease.