He told the club's official website: "I have been with the club for three years now and after the £2million offer from Wolverhampton Wanderers was rejected I got my head down and helped us get promoted to the Championship.
"I was made club captain and then dropped for the biggest game of my career at Newcastle United. After the game I spoke with the chairman [Darragh MacAnthony] about how upset I was. I did not hand in a transfer request at this time as he asked me to back him.
"I decided again to try and get on with things as the chairman, staff and supporters have been very dear to my heart but I just can't get over the way I was treated and now feel these factors, my age and my family commitments make it the right time to move clubs."