Thornton's agent, Anton Thun, confirmed in an email to The Canadian Press that his client was appealing. The NHL Players' Association announced it had informed the league of that plan before the 48-hour deadline to do so passed.
The appeal first goes to NHL commissioner Gary Bettman, per the collective bargaining agreement between the league and players' union, who will provide a written judgment.
Buffalo Sabres forward Patrick Kaleta is the only player to use this appeals process under the new CBA, and no player has gone to the neutral arbitrator. Bettman upheld Kaleta's 10-game suspension for an illegal hit on Columbus Blue Jackets defenceman Jack Johnston.
On Saturday, NHL disciplinarian Brendan Shanahan meted out the punishment to Thornton, who pulled Penguins defenceman Brooks Orpik down during a Dec. 7 game and punched him numerous times in the head, resulting in a concussion.
"I will be consulting with the Bruins, my representantion [agent Anton Thun] and the NHLPA about next steps," Thornton said through a team news release after the NHL announced the suspension.
Vice president of player safety Brendan Shanahan said it was not a spontaneous action by Thornton because he tried to confront Orpik earlier after Orpik injured Bruins winger Loui Eriksson with a hit.