My vitamin D levels were never checked. I ended up with osteoporosis...a moderate case. I was given Evista and take a lot of calcium with D in it. When I had a second bone scan two years later, my bones weren't any better even on this regimen! THAT'S when he checked my vitamin D and found a severe deficiency! You need D to absorb the calcium so I wasn't absorbing it at all.
I still have fog but it does seem a little better. I'm just barely in the low levels of normal. Normal is 32-100 at this lab and I'm a whopping 33! 
So, I'm taking more D...5,200 IU's a day.
I do think this is an extremely important vitamin and they are finding that so many people are deficient because of using sun screen all of the time. It has helped me with my pain and fatigue and it is good for the cardiovascular system, too. When I find things about it, I post it here for the members to consider.
Sherrine