Quoted from ruthiegirl

It's not so much about the dose you're taking orally; it's about the blood levels of vitamin D. I was taking 4,000 iu daily when a blood test confirmed low vitamin D; either I wasn't absorbing the vitamin well, or my body used it up too fast. Upping my dose (10,000 iu for a while, with frequent lab tests to monitor my levels) brought it up within a few months.

I don't know if Research continues along these lines, but a number of years ago (5 or so?) the D2 vs. D3 issue was being studied, positing that D2 is the stored form of Vitamin D, and that D3 is the circulating form. It was being recommended that large doses of D3 be taken to tackle an acute infection, for instance, but that D2 be taken to replenish diminished stores, i.e., remedy an outright deficiency.

Surf around if this interests you.