My doctor recently told me that I'm not getting enough vitamin D, and that I need to take a supplement. Vitamin D is one of the ones that it's possible to overdose on, so I looked it up. Overdosing on D, while possible, isn't that easy to do. And in fact, many people actually have a deficiency.
This website can tell you just about anything you want to know about vitamin D:
Learn the Symptoms of Vitamin D Deficiency Before It’s Too Late!
It has lots of links. One of them was this one:
Which kind of vitamin D should I take?
It says, among other things, that recent studies have show that the maximum vitamin D levels that have previously been recommended as normal can and should be considerably higher.
This other site was also very informative too:
vitamin d deficiency symptoms guide