It explains the metabolism of Vit D.

http://www.merck.com/mmpe/sec01/ch004/ch004k.html

This is a difficult subject because the past studies have been confusing.
This is why I gave you Dr. Vieth's paper.

The D used by doctors in that shotgun high dose is D2, and it has a different
potency (about 1/2 as useful as D3) and more toxic potential than D3, which appears to be more useful.

There are a very few online sources for D3 in 50,000 units but I prefer to not post them. I think it is best to get tested first.

Like other metabolic disorders, hyperparathyroidism, and errors in D metabolism are not easy to understand. I don't expect doctors have a good handle on it yet. They are testing, but I am not convinced they are treating properly.
I saw all kinds of doses all over the place in the nursing home provider I used to work at.

I would suggest that people reading here, simply ask their doctors what is the difference between RX D they are offering and D3 over the counter. The answer to that question will reveal their ability and understanding.
If they tell you their RX is better... well... that is disappointing right there!