You will be very lucky if you find a doctor who knows anything about Vitamin D3 that is worth repeating. Mostly they repeat out of date information that your basic common sense should be sufficient to demonstrate is utter rubbish.
Remember your body will, if you take your clothes off and go outside and lay down for 20minutes when your shadow is shorter than your height, make between 4000iu-10,000iu of D3.
Doctors on the whole will tell you 200iu-400iu daily is all you need and that 2000iu of the same stuff your skin makes in such quantity may be toxic.
We would not accept such reluctance to keep up to date with the latest research from the chap who fixes our PC so why we allow the chaps who fix our bodies to stay 20 yrs out of date with their BS information beats me. But Just how much good has medicine done over the years, and how much harm does it continue to do? Shows things have always been this way and nothing changes.