Gigi Hadid responded to critics of her weight in a series of tweets on Sunday night. The model, who is suffering from a thyroid condition called Hashimoto's Disease, explained that her changing body type was a result of the illness.

"For those of you so determined to come up w why my body has changed over the years, you may not know that when I started @ 17 I was not yet diagnosed w/Hashimoto’s disease; those of u who called me 'too big for the industry' were seeing inflammation & water retention due to that," she said in her first tweet.

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Hashimoto's disease is a disorder where your immune system attacks your thyroid, leading to chronic inflammation, according to the American Thyroid Association. Over time, it results in the thyroid's inability to produce ample hormones and, eventually, leads to an underactive thyroid. There are no specific symptoms unique to the illness, according to the ATA, but some signs include swelling at the front of the throat, a puffy face, and unexpected weight gain, according to Mayo Clinic.

"Over the last few years I’ve been properly medicated to help symptoms including those, as well as extreme fatigue, metabolism issues, body’s ability to retain heat, etc," Hadid continued. "I was also part of a holistic medical trial that helped my thyroid levels balance out."

The 22-year-old added, "Although stress & excessive travel can also affect the body, I have always eaten the same, my body just handles it differently now that my health is better. I may be “too skinny” for u, honestly this skinny isn’t what I want to be, but I feel healthier internally and am still learning and growing with my body everyday, as everyone is."

Gigi ended on a powerful note, stating she would no longer "explain the way my body looks" to her critics and clarified that "drugs are not my thing."

And she urged her fellow social media users to "learn to have more empathy for others."

Hadid previously opened up about her diagnosis in December 2016. "My metabolism actually changed like crazy this year. 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 at the time.

Because of her changing body, Hadid actually aimed to build muscle rather than shed weight while training for the 2016 Victoria's Secret Fashion Show. "I just want to have muscles in the right place," she said, "And if my butt can get a little perkier, then that's good."

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Erica Gonzales

Erica Gonzales is the Senior Culture Editor at ELLE.com, where she oversees coverage on TV, movies, music, books, and more. She was previously an editor at HarpersBAZAAR.com. There is a 75 percent chance she's listening to Lorde right now.