Van Valkengoed's main achievement is winning the bronze medal in the 200 m breaststroke at the European Short Course Swimming Championships 2003 in Dublin, Ireland.

After the disappointing results in Montreal at the 2005 World Aquatics Championships it appeared that Thijs had been overtraining himself for a longer period of time.

After his overtraining he decided to move back from Amsterdam to Lelystad and started to train with his sister with the goal to qualify themselves for the 2008 European Aquatics Championships in Eindhoven.

At the end of 2007 both Thijs and Jolijn van Valkengoed reached their goal and qualified for the European Championship in their home country.

During the European championships Thijs lowered his own national record at the 100 m breaststroke to 1:01.22 and instead qualified for his second Olympics.