Fans couldn't help but notice Gigi Hadid's super-lean physique when she walked the runway at the Victoria's Secret fashion show in Paris. Determined to quash rumours she intentionally slimmed down for the event, the model has now revealed her secret battle with autoimmune condition Hashimoto's disease.

Speaking to Elle magazine during a promotion pitstop for her new Reebok campaign , the 21-year-old star revealed that her weight had plummeted in recent months because of medication that interferes with her metabolism. "I have Hashimoto's disease, it's a thyroid disease, and it's now been two years since taking the medication for it," she said.

"So for the VS 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."

Avatar actress Zoe Saldana also suffers from the condition, which causes the immune system to attack the thyroid gland and destroy thyroid tissues. This often leads to hypothyroidism and can cause weight gain and depression.

In a bid to manage her condition Hadid turns to exercise. "What's great about boxing is that I couldn't come to New York and sign up for Equinox, and go running. It was not good. So, I knew I needed to find something that gave me a coach, gave me a family — so that when I walk in, it's like my big brothers.

"They're like 'what up!' They don't care about who I am. That's what I needed, and they don't care about what my body looks like. They care that I give a good punch, and that's how I want to be judged. That's what makes me proud of myself."

Hadid's revelation comes weeks after her mother Yolanda addressed her battle her with Lyme's disease – a bacterial infection spread to humans by infected ticks. She revealed that Lyme diseases had robbed Bella of her dreams of being an Olympic-level equestrian rider.

"She's always tired," Yolanda said on the Good Day New York show. "She could sleep for 10 hours and wake up [and say], 'I'm still tired.' In the younger generation, it comes up like anxiety, severe exhaustion, joint pain, flu-like symptoms and they just keep struggling over it."