  Let $G$ be a simple and finite graph without isolated vertices. In this paper
we introduce and study a new degree sequence derived invariant called the
\emph{sub-total domination number}, denoted $\sub_t(G)$. In particular, we show
that $\sub_t(G)$ serves as a lower bound on $\gamma_t(G)$, where $\gamma_t(G)$
denotes the heavily studied \emph{total domination number} of $G$.
