Thanks for the info. mrsd! I haven't looked at the links yet but I certainly will. I'm not doing it myself; I just asked the first doctor I saw after hearing about this because knowing that too much can be toxic, this stumped me. There are at least 3 people with MS whose doctors have prescribed this for them. I will direct them to your post so that they can look at the links as well.

Two more question about D, if you don't mind. I believe that both the liver AND the kidney are implicated in converting the vitamin, correct? If this is the case, when checking for toxicity, should the kidneys be checked as well, or just the liver?

Also, if the problem in low levels is determined to be absorption, could the underlying problem in fact be something to do with the kidneys or liver?