Gigi Hadid blames thyroid disease medication for thinner figure

BEFORE AND AFTER: Gigi Hadid has disclosed she suffers from thyroid condition Hashimoto's disease and that the medication she takes causes her to lose a lot of weight. Photos / AP

Gigi Hadid has revealed she suffers from Hashimoto's disease, a condition that affects the thyroid.

The model, 21, told Elle that she's been getting treatment for hypothyroidism, where the metabolism slows down sometimes causing weight gain and depression, for two years, and says its the medication that's responsible for her newly skinny figure.

"My metabolism actually changed like crazy this year," she said. "So for the VS show I didn't want to lose any more weight."

Model Gigi Hadid displays a creation during the Victoria's Secret Fashion Show inside the Grand Palais, in Paris. Photo / AP

Gigi's revelation comes on the back of her mother Yolanda's highly publicised battle with Lyme disease, which her younger sister Bella, also a model, and younger brother Anwar, also suffer from.

On Wednesday in New York, the blonde beauty showed off her thin frame in a crop top and leggings at a Reebok event.

Her flat tummy and sinewy arms were visible as she threw out punches during an on-stage workout.

Her admission that she struggles not to lose weight comes just a year after she walked the runway at Tommy Hilfiger's Fall 12015 show dressed in a poncho because, the designer said in an interview with Yahoo! Style last month, the casting director thought she was 'not quite as thin' as the other models.

And judging from photos of her in a bikini taken as she celebrated her 20th birthday in April 2015, there's no doubt Gigi has shed quite a few pounds in the past 12 months or so.

Despite her diagnosis, however, the model proudly tells Elle: "You know, I've never taken a sick day."

Model Gigi Hadid displays a creation during the Victoria's Secret Fashion Show inside the Grand Palais, in Paris. Photo / AP

"I feel that pressure of not taking time for yourself because you don't want it to affect your professionalism," she added.

According to the American Thyroid Association, Hashimoto's Thyroiditis is most common in middle aged women, although it can occur at any age and can also affect men.

While it may slow the metabolism, the most common symptoms are feeling the cold and depression, and once treatment with synthetic thyroid replacement begins, the body usually returns to the weight it was before the condition manifested itself.

"Since hypothyroidism usually develops over a long period of time, it is fairly common to find that there is no significant weight loss after successful treatment of hypothyroidism," the ATA website states.

Model Gigi Hadid, center, displays a creation during the Victoria's Secret Fashion Show inside the Grand Palais, in Paris. Photo / AP

Meanwhile, on Thursday Gigi was spotted stocking up on items from Victoria's Secret on a shopping spree in NYC.

The 21-year-old was seen returning to her apartment carrying several bright pink VS store bags full of her purchases.

Gigi had stepped out in powder blue stiletto-heeled ankle boots with laces up the front that she paired with high-waisted jeans with the bottoms trolled up above the boots.

She added a black top and a wraparound black winter coat and wore her hair tied up into a ponytail.