jesseam was kind enough to share his comments about vitamin D for TBI on my other thread. Comments on this thread finally jarred my memory. Vitamin D3 is the supplement that I buy, which is also called cholecalciferol. Vitamin D2, aka ergocalciferol, has to be converted to D2 in the kidneys. I take 1000 IU(international units) of D3 in a tablet and 400 IU of D2 in a multivitamin daily.
People have become sick taking too much Vitamin D. The blood test for it is serum 25(OH)D, aka 25-hydroxyvitamin D. You can find the source for the test by searching the keyword vitamind in this forum. It's on of of my threads in the last month or so. The keyword tbi will find many threads on traumatic brain injury in this forum.