I don't take such a high dose (I was on 2000IU a day and got the point where it was causing my heart to race and I had to back off), but I take a vitamin D supp

I don't take such a high dose (I was on 2000IU a day and got the point where it was causing my heart to race and I had to back off), but I take a vitamin D supplement where a 1000IU dose is 3 drops of liquid from a dropper. It would be easy (although very expensive, and IMO not advisable) to take a large dose via this method. I have a suspicion most people taking such a high dose (over an extended period of time) must be using a poor quality of vitamin D that means they're not actually absorbing that much. But that's just speculation.