I have managed to trace the name back to the 1400's and also an alternate spelling. Thielen. I am told it means son (or descendant) of a Thel, Thele or Thiele. The name originates in the Rhineland and there were a few in Westphalia. The earliest occurence, oddly, was in Upper Franconia when one of the sons of a forge owner (either von Dolein or von Theler) took the name von Thelen. However, the name appears to have died out in that part of Germany and there is no connection to the name in the West of Germany.
My question is whether there is some meaning to the stem name Thel, Thele or Thiel.