The Steiner Tree version that I am considering is the following: given an undirected weighted graph $G=(V,E)$ and a subset of vertices $T \subseteq V$, find a minimum tree that connects all the vertices in $T$ which possibly contains vertices outside $T$.
I know that both TSP and Steiner Tree are NP-hard and there are approximation algorithms for these problems that have an approximation guarantee of $2$.
I was wondering whether there is any reduction from the Steiner Tree to TSP?