Posthuma was founded in 2004 with Team Rabobank professional and got there a contract until 2006. Previously, he drove two years in the youth team of Rabobank. He proved himself in his specialty discipline time trial in the Vuelta a España with a tenth and an eleventh place. His first professional victory was in his first ProTour race Paris-Nice after a successful breakaway. In 2005 he took part in the Tour de France for the first time.
In his second appearance in 2006 he wore during the first stage of the white jersey as the best youngster.
On 18 April 2007 accident Posthuma in the Munsterland in a training accident and injured his hard case.
But after his injury he picked up again in July, the Sachsen-Tour to decide for themselves. For more tour victories he achieved in the coming years in the three days of De Panne, the Tour de Luxembourg and the Ruta del Sol.
After the 2012 season, he finished his career.
- Prologue Tour de Normandie
- Appreciation Olympia's Tour
- Appreciation International Thüringen Tour
- Appreciation and a stage Circuit de Lorraine
- A stage of Paris-Nice
- Grote Prijs Jef shearing
- Appreciation and a stage Sachsen-Tour
- Appreciation and a stage Three Days of De Panne
- Appreciation Tour de Luxembourg
- Appreciation Ruta del Sol
- A stage in Tour of Austria
- 2002 Rabobank TT3
- 2003 Rabobank TT3
- 2004 Rabobank
- 2005 Rabobank
- 2006 Rabobank
- 2007 Rabobank
- 2008 Rabobank
- 2009 Rabobank
- 2010 Rabobank
- 2011 Leopard Trek
- 2012 RadioShack -Nissan