I have never been on that exact combo myself, but I've been on Li and valproic acid separately. It would make sense if lithium is already affecting your thyroid and you stayed on it at the same dose, it would continue to affect your thyroid and you would also have to start taking something for it. Also, valproic acid can cause liver damage if higher doses are taken for a long amount of time so, like lithium, it's important to get your levels checked frequently. Taking them both as far as I know doesn't lessen the side effects from either.