Having received his senior debut for Denmark, Würtz captained the Danish under-21 national team, as it qualified for the 2006 European Under-21 Championship in May 2006. He was the Danish under-21 team captain at the tournament, playing full-time in Denmark's three matches, and scored a goal against the Italian u-21s. Following Denmark's exit from the tournament, he ended his under-21 national team career due to age restrictions, having played 29 under-21 national team matches in all. He was nominated for the Danish league national team squad, a selection of the best Danish Superliga players, to tour U.S., El Salvador and Honduras in January 2007. He was named team captain, and started in all three games of the tour.