After spending his entire professional career with the Rabobank teams, the 31-year-old signed with Leopard Trek for 2011-2012, and remained with the team through its merger with RadioShack.
Posthuma was part of the Leopard Trek Tour de France team which put both Andy and Fränk Schleck on the final podium behind Cadel Evans in 2011. However, his important role as a domestique was of little value in a system which puts UCI points as the most critical factor in any team's bid for the WorldTour.
"I have been negotiating with different teams such Vacansoleil, but they had no room. Additionally, there are many teams that need ProTour points and, to be honest, I have none. Teams need points in order to remain in the top tier," Posthuma said.
He said that the Leopard Trek team ended badly when the merger happened and it meant the exit of many of the team's younger riders. While he survived the cut, Posthuma said the move pushed him down from the 'A-team' to a C and then "an F-rider". "Of course I was sick at some key moments, but there were clearly other factors. Eventually it became demotivating to train so hard on the road, behind the scooter, for the big races and then the big races never come."
Posthuma admitted that his lack of motivation and two young children at home led him to simply give up his search for a new team, and find a new career rather than take a step back to a smaller team.
"I'm ready for a new challenge," he said. "I have a lot of positive energy for the future. Last week I signed up for the half-marathon in Enschede.
"For nearly a decade I've lived for cycling alone. It may sound silly but there is a whole new world opening up for me."
Posthuma listed his neo-pro stage victory in Paris-Nice and his first time riding the Alpe d'Huez and Paris stages of the Tour de France as some of his best memories from his career. But it was his recovery from a serious leg injury from being hit by a car while training in early 2007 and subsequent victory in the Sachsen Tour that was his most satisfying time.
After being told he might never ride again, Posthuma said, "Eventually I won the Sachsen Tour, which was quite a relief. Normally I'm not so emotional, but at that moment I shed some tears. It is a sport that gives you so much, but it also demands a lot in return."
Posthuma went on to win the overall Driedaagse De Panne - Koksijde and Tour of Luxembourg in 2008, Ruta del Sol in 2009, and a stage in the 2010 Tour of Austria.