The former Kiwis rugby league international and 2008 World Cup winner Lance Hohaia will seek medical advice about potential long-term health problems after announcing his immediate retirement from rugby league.
He retiring citing "recurrent post-match concussion-type symptoms".
Hohaia's decision came a little more than six months since being knocked unconscious by Wigan forward Ben Flower in St Helens' Super League grand final victory at Old Trafford.
Saints declined to offer an explanation for the surprise development other than to say it was the player's own decision - but Hohaia told the BBC he is seeking medical advice.
"I've been experiencing recurrent post match concussion-type symptoms since returning to play," the 32-year-old said.
"I am seeking medical advice to understand any potential effects to my long-term health."
In a statement, the club said: "St Helens announce that Lance Hohaia has informed us of his retirement from rugby league and will therefore not play for the club again.
"Lance has been a valued and integral member of the squad for the last three-and- a-half years and we thank him for his service.
"We wish him and his family well in their life after rugby league with St Helens."
Hohaia's Super League club career reached a high in October when he helped St Helens reach the grand final but his involvement lasted less than two minutes after he was twice punched by Flower in the opening moments of the match.
Flower, who had reacted to a swinging arm from Hohaia, was handed a six-month ban, which ended earlier this month, while the St Helens halfback was given a one-game ban for provoking the Welsh international.
Hohaia recovered from his knock in time to collect his winners' ring after the final whistle.
He made 81 appearances for Saints in a variety of positions, including fullback, hooker and halfback.
His final appearance in Super League was in Saints' 11-8 win at Huddersfield on April 12.
He has been studying a diploma in management as he prepares for life after rugby and recently spoke about the prospect of retirement.
At the 2008 World Cup, Hohaia started at fullback and scored a try in the final as New Zealand upset Australia to win the competition for the first time.
Hohaia was out of contract at the end of the season and, unlikely to be offered a new deal, had spoken about moving to the United States to start a new life with his US-born wife and two young boys, who were both born in England.