A function f : V (G) → {−1, 1} defined on the vertices of a graph G is a signed total dominating function (STDF) if the sum of its function values over any open neighborhood is at least oneIn this paper first we find lower and upper bounds for the global signed total domination number of a graph. Then we prove that if T is a tree of order n ≥ 4 with ∆(T ) ≤ n − 2, then γ gst (T ) ≤ γ st (T ) + 4. We characterize all the trees which satisfy the equality. We also characterize all trees T of order n ≥ 4, ∆(T ) ≤ n − 2 and γ gst (T ) = γ st (T ) + 2.