It's not uncommon in my experience for lithium to be paired with another medication, like valproic acid. I have a similar med situation with lithium and another med, lamotrigine. It works really well for me! Like you said though, it does depend on the person.

As far as health effects and dosages go, I know with lithium the really bad effects happen when the blood levels are regularly above a certain point. I fairly certain valproic acid functions the same way.

That said, definitely keep an eye on the thyroid if you're already having trouble with it.