Concussion forces Reddish to retire
Commenting on his retirement, Reddish said: "Having to retire is extremely difficult, but given the medical advice I have received, it's the best decision long term for me and my young family.
"I thoroughly enjoyed living in England and I am sad to cut my time short. I wish the boys all the best for the up-coming season and beyond,"
Reddish joined Harlequins in November 2016 from Highlanders where he won the Super Rugby title with the franchise in 2015.
The New Zealander made five appearances for Harlequins and scored one try, in the win away at Bristol Rugby.
Reddish also started for Harlequins in the second row against the Maori All Blacks in the 150th Anniversary Challenge match at The Stoop in November 2016.
Before joining Highlanders, Reddish played four years at the Hurricanes and played provincial rugby for Wellington between 2009 and 2016.
Harlequins' Director of Rugby John Kingston added: "It is very sad that Mark has been forced to retire due to concussion. It is absolutely vital that recognition is taken in such circumstances and I am in no doubt that the right decision has been made.
"Myself and all at Harlequins would like to take this opportunity to wish Mark and his family the very best in their future endeavours,"
