Thirlwall was treated in hospital aged 16 and says she was tormented by an “anorexia angel” on her shoulder.
The northern pop star went on to make a full recovery and was discharged just weeks before auditioning on "The X Factor" in 2011.
Now Thirlwall is determined to help youngsters battling eating disorders and recently visited patients at a hospital in Sheffield to tell them about her own experiences.
The emotional day was recorded for "BBC Sounds Life Hacks" podcast "I’ve Been There: Jade Thirlwall & Anorexia."
Reflecting on her own battle with the mental health condition, Thirlwall told host Katie Thistleton: “Anorexia was my own dark secret and I guess I was sort of satisfied with that.
“It was my own thing that I could do to myself and nobody knew about it.
“I obviously became very small and my ribs were sticking out. I was very gaunt and I used to wear a lot of baggy clothes to hide that.
“In my head, I felt so down and depressed about everything that was going on in my life, I really just wanted to sort of waste away.
“I got in a really horrible state. The turning point — and the reason why I first told my counselor — was when I got tired of hating myself so much.
“The second turning point for me was when I was at the hospital and the doctors told me that I would die if I kept doing it.
“To hear somebody say that to you is actually quite scary and I started to realize how damaging it was for my family.
“I’d become so selfish with how I felt about myself I forgot that I had family and friends who were also really hurting because of what I was doing.
“It sounds really weird, but I saw anorexia like an angel on my shoulder. Anorexia for me was control, and if I was controlling something then I was winning.
“It wasn’t until I had therapy about it that I realized anorexia was actually the devil on my shoulder. That it wasn’t my friend.
“I really struggled to understand that at first, because I was so isolated and didn’t talk to anyone.
“I’d got so used to hearing that voice telling me, ‘Don’t eat that’ or ‘Don’t look in the mirror’, ‘You’re still ugly, you still have a long way to go’.
“It took a long time to realize that voice wasn’t good for me anymore.”
Thirlwall, who grew up with parents James and Norma and brother Karl in South Shields, Tyne and Wear, believes her anorexia began after her granddad died when she was 13.
The number of teenage girls diagnosed with anorexia has rapidly increased in recent years and 1.25 million people in Britain are currently living with an eating disorder.
Thirlwall listened to an interview with 14-year-old anorexia sufferers Marissa and Megan.
Marissa has battled the condition for three years and only sought help after she came across a recovering anorexic’s Instagram post discussing how her periods had stopped.
Thirlwall tells podcast host Thistleton, 30: “It’s really scary when you haven’t spoken to anyone yet and you know you’re hurting yourself but you don’t understand the seriousness of it.
“My period stopped as well and I thought, ‘Oh well, it’s just a side effect of doing what I’m doing’.
“I didn’t realize that I was gradually destroying my body, more and more.
"It sounds quite twisted but I was — not proud of myself — but I knew I was having the effect that I wanted on my body.
“I was destroying myself and that’s what I intended to do.
“But when you’re actually told by someone, especially a doctor, that you’re going to die if you keep doing what you’re doing — especially if someone says that in front of your mom, or someone that you really love — it is really a hard moment.”
Megan has struggled with anorexia for two years. She told Thistleton she would feel so guilty about eating that she punished herself by sleeping on the floor.
Thirlwall’s visit was arranged as a surprise for Little Mix fans Marissa and Megan, who squeal with delight when she walks into the room.
The pair quiz Thirlwall on her own struggles with anorexia, in particular her worst moment. She tells them: “Probably the first day I had to go to the hospital. I remember my mom and dad had a bit of a breakdown.”
Following regular therapy and visits to the hospital, Jade was discharged just weeks before auditioning for "The X Factor" in summer 2011, aged 18.
Asked how she avoided a relapse, Thirlwall replies: “I knew I would ruin my dream of becoming a singer.”
She adds: “Being famous still has a lot of downsides — like people constantly talk about the way I look if I put a little bit of weight on, and there are unflattering photos.
"Honestly, I still have times where I’ll feel a bit sad or down about something, but I now don’t associate that with eating anymore. I don’t punish myself in that way.”
After the visit, Marissa says: “It just shows you that there is life after the hospital and you can recover.”
And Megan, who has since been discharged, adds: “It doesn’t stop you from reaching your goals. Jade said she had a goal in her mind and she got it.”
While 46 percent of anorexia sufferers make a full recovery, it remains the biggest killer of all mental health conditions. But Thirlwall’s story shows there is hope for youngsters out there battling it.
"Life Hacks – I’ve Been There: Jade Thirlwall & Anorexia," produced by Amelia Ellis, is released Sunday on BBC Sounds. Listen here.
This article originally appeared in The Sun.