Heavy metals are mostly toxic. What constitutes heavy, metal and toxic is subject to scrutiny. There are plenty of light toxic elements, such as Fluoride. The definition of metal also varies. Most things can be toxic in sufficient quantity, and responses to various toxins is highly individual. Some toxic agents such as Bromine have recently demonstrated a natural usage at very low levels.  This might be true for most elements (minerals) since life co-evolved with them and often utilizes their features to power an enzyme or process.