[jamesisaac]

That makes sense with your example - but I really don't think suicide can be seen as a symptom of lack of lithium in the same way as sickness from a lack of iodine.

What is there to show that "this same story holds for any vitamin or mineral"?! Surely there are many which have no positive effect on the body, where zero would be the best dosage. What law of biology suggests that every mineral is in some way beneficial to the human body?

Lithium happens to have an effect on moods in the brain in such a way that suicide is less likely - but the mood dampening effects may well diminish people's positive emotions too (for example). For someone with no mental health issues or risk of suicide, this could well be an overall negative effect, and therefore makes no sense to force on the entire population.