AceShowbiz - Imagine Dragons lead vocalist Dan Reynolds has opened up about his newly svelte physique on social media.

The 30-year-old singer took to Instagram earlier this week (begins July 2) to show off his incredible body transformation, which has seen him shed the pounds and tone up dramatically.

The "Thunder" hitmaker told fans he embarked on an intense diet and exercise regime after suffering from autoimmune disease Ankylosing spondylitis (AS) for over a decade, which reportedly left him in agonising pain and discomfort. Sharing a topless photo of himself to his 416,000 followers, the artist revealed he's never felt healthier and thanked his personal trainer for turning his life around.

"I've spent a decade fighting an auto immune disease that inflamed all my joints (AS). I now live pain free, am in full remission and am the healthiest I've ever been," he gushed. "I owe it to this man @thor345622 (trainer Brad Feinberg). Correct diet, vitamins and exercise. I'll share it all with you soon. It's been life changing for me. Xx."

I’ve spent a decade fighting an auto immune disease that inflamed all my joints (AS). I now live pain free, am in full remission and am the healthiest I’ve ever been. I owe it to this man @thor345622 Correct diet, vitamins and exercise. I’ll share it all with you soon. It’s been life changing for me. Xx

A post shared by Daniel Coulter Reynolds (@danreynolds) on Jul 1, 2018 at 8:34am PDT

The "Radioactive" singer then shared his impressive before and after photos, showcasing the star's newly formed six-pack.

"just a few months of reworking my entire lifestyle/diet/exercise w (Brad Feinberg) and I have zero inflammation now," he captioned the post. "These before and after pictures say it all. Sorry for the corny exercise post but truly changed my life and healed years of pain. Unbelievably grateful for health."

just a few months of reworking my entire lifestyle/diet/exercise w @thor345622 and I have zero inflammation now. These before and after pictures say it all. Sorry for the corny exercise post but truly changed my life and healed years of pain. Unbelievably grateful for health.

A post shared by Daniel Coulter Reynolds (@danreynolds) on Jul 2, 2018 at 9:21am PDT

Fans were quick to congratulate the star on his achievements and celebrate his good health. "So incredibly happy for you," enthused one fan, while another added: "Absolutely amazing results; both internally and externally!!! #epicpost."

AS is a type of arthritis that typically causes long-term inflammation to the joints in the spine. While there is believed to be no cure for the disease, experts recommend exercise to manage its symptoms.