Victoria Justice is battling Hashimoto's disease.
The 22-year-old actress was diagnosed with the condition, in which the immune system attacks the thyroid, a few years ago and has finally gotten it under control after weight fluctuation and skin breakouts.
She told Health magazine: "A few years ago, I was diagnosed with Hashimoto's, which is a thyroid autoimmune disorder [that can interfere with thyroid function and affect metabolism].
"They had put me on this thyroid medication, and I think the levels were too high for me because it caused my skin to break out. All my life I've always had great skin, so it was really devastating. Also, at first, I was losing a lot of weight, then I went on tour and I started gaining weight, and it was the most I'd ever gained in my life. I was, like, 115 pounds. Then when I filmed a movie last summer, I actually went under 100 pounds. It was crazy, but things have leveled out, and I'm back to normal.
"I still have Hashimoto's. But my numbers are back to normal, and I'm not on medication. I feel good."
Victoria revealed she had tried a number of things, including a strict diet, to get the disease under control.
She said: "This Korean acupuncturist put me on a super-strict diet, where I was a pescatarian [eating fish but no other meat]. I also couldn't eat any sugar, wheat or dairy. I did that for about two months, and it was so intense."
And she admitted scrutiny about her appearance was hurtful.
She said: "It definitely did affect my confidence, and it made me a little bit more self-conscious. But at the end of the day, I had the support of my family and friends. I knew that the people around me had my back. Now I really empathise with people who have issues with their skin, because it can get you down."