The former Germany international, who overcame a number of serious injuries during his career, admits that his body is no longer up to the rigours of professional football
The former Germany international made his name at Borussia Dortmund but moved to Real Madrid in 2007, only to see his time in Spain blighted by injury problems.
However, as Metzelder explained, he had done well to make it even that far into his career having suffered a serious Achilles tendon injury during his time at the Westfalenstadion (now Signal Iduna Park).
"I was very close to having to end my career back then - I still have a letter from my health insurance company declaring I was unable to play sport again," the 33-year-old centre-half told Schalke's official website.
"I was only 23 but I fought my way back and experienced many great years, including participating in World Cup 2006 and Euro 2008.
"I look back with gratitude on a fantastic career. I passed my A-levels when I was 19 and, two weeks later, I was making my debut in the Bundesliga and I was capped for the first time just a year after.
"When I look back, I see nothing which could have gone any better. Even the injuries were part of my story.
"I am just being honest with myself and I feel that I am no longer physically able to play consistently at this level. And, after 13 years, the fire inside me, which is necessary for this job, has burnt out."
Metzelder added that memories of Germany's stirring run to the semi-finals of World Cup 2006 will forever remain vivid as he looks back on his playing days.
"That is the highlight of my career," he said. "Those seven weeks, from the first training camp with Jurgen Klinsmann in Sardinia to the reception in front of a million fans in front of the Brandenburg Gate was the greatest time I ever experienced as a professional footballer.
"It was amazing and certainly none of us will ever forget it. I wrote a diary at the time and maybe I should publish it sooner or later!"