Promising Toronto FC youngster Zac Herold has been forced into premature retirement because of a heart condition.

The 17-year old defender has been diagnosed with Hypertrophic cardiomyopathy which causes abnormal thickening of a part of the heart muscle and has had to announced his retirement for risk of developing a potentially fatal irregular heart rhythm or arrhythmia.

"Announcing my retirement from soccer at this age is something I never, ever thought would happen to me," Herold said in a statement. "This news was very hard to take but I know it's the best decision for me and my health.

"My parents have been with me every step of the way and it means so much to me that I'm able to retire as a Toronto FC player."

Herold will be honoured when Toronto play New England Revolution at home this weekend, confirmed by general manager Mo Johnston.

"Our hearts go out to Zac and his family," Johnston said. "It's a very difficult situation.

"We felt it was the right thing to do by honouring him in front of our fans and give them a chance to show their support."