The 21-year-old supermodel has revealed her metabolism has changed "like crazy" over the past year because of the condition, which is a thyroid disease and means the body fights against its own tissue, and has been taking medication to slow the disease down since she was diagnosed.
Speaking to Elle.com about the condition, the blonde beauty said: "My metabolism actually changed like crazy this year. I have Hashimoto's disease. It's a thyroid disease.
"It's now been two years since taking the medication for it."
And the catwalk icon has revealed due to the illness, she was determined not to drop anymore weight prior to gracing the runway for the annual Victoria's Secret Fashion Show, which took place in Paris, France, on Monday (05.12.16).
She explained: "So for the [Victoria's Secret] show I didn't want to lose any more weight."
And the star has hinted she is frustrated by the disease because all she wants is to have "muscles in the right place" instead of having her body attic her toned limbs.
She said: "I just want to have muscles in the right place, and if my butt can get a little perkier, then that's good."
Meanwhile Gigi - who is the daughter of Yolanda Hadid, who opened up about her battle with Lyme disease last year - has revealed when times get tough she thinks about her high school coaches and what their opinion of her might be now.
She explained: "[During stressful times] I try to sit myself down and say, 'If it wasn't for the world's opinion, do I think I did a good job?' 'Would my coaches in high school be proud of me?' Those little things I try and think of. When my high school volleyball coach will text me and say, 'I think that was bad ass.' That's what makes me happy."