Get it, girl! Kelly Clarkson sounded pitch-perfect as she belted out a medley of her greatest hits at the Radio Disney Music Awards on June 22. But it wasn’t just her voice that had fans cheering. Rocking a clingy sequined dress, sky-high heels, and a glowing complexion, a slimmed down Kelly looked healthier — and happier — than she’s been in years. "Kelly’s lost 40 pounds and she feels amazing," an insider told Life & Style in the magazine's latest issue, on newsstands now. "She really is in a great place."

She didn’t get there by bowing to peer pressure and starving herself. The "Stronger" singer was battling an autoimmune disease and a thyroid issue — and wound up changing her diet purely for her health, Life & Style revealed. "She kept it a secret from fans because she wanted to get it under control first," revealed a source. "Now she feels like she can be an advocate for a healthier lifestyle. This isn’t something she did to get skinny. She did it for her family and for herself."

It took her a while to get there. After years of yo-yoing weight and mood swings, Kelly learned at a doctor’s appointment that she had a thyroid and autoimmune disorder. Though she treated it with medication, Kelly still wasn't feeling her best, so she set about trying to heal herself with a healthy diet instead, taking cues from the book The Plant Paradox by Steven Gundry. "It’s worked for me," Kelly, 36, revealed. "It’s basically treating medicine like medicine and not like candy, and it’s about food being food for fuel and not so toxic. We spray everything [or] pump it with antibiotics."

Now she tries to avoid that whenever possible. Though she admits her all-organic, plant-based diet is "very expensive," Kelly is now completely off her meds. And she has more energy than ever before. "At first it was hard for her and she struggled with changing her diet," the source said of Kelly, who still allows herself cheats like homemade chocolate chip cookies. "But as soon as she started feeling better, she knew she was on the right path."

These days, a revitalized Kelly is enjoying new successes in her career. She recently won her first season on The Voice with contestant Brynn Cartelli and soon starts work on Season 15, returning this fall. "She would be a coach every season if they let her," said the source. "But she’s also got a lot more lined up — deals and specials with NBC. Kelly now feels like she can do anything."

But it’s her family that comes first. As busy as she is with work, she and her husband, talent manager Brandon Blackstock, 41, reserve most of their energy for daughter River, 4, and son Remington, 2. "We strive to make sure that they feel like they count, they matter, that they’re loved and appreciated," Kelly revealed. "We have a crazy life, and they get a lot of great opportunities because of it. But they are our main focus. And they’re doing great!"