Dr David Perlmutter is a neurologist who pratices "functional medicine" and makes a lot of articles and videos. The articles on his website often refer to scientific literature and you get get a list of articles by doing a search on his site: https://www.drperlmutter.com/?s=vitamin+d

You will see a list of articles what Vitamin D does to prevent and heal diseases.

I am taking 15.000 IU/day and have regular blood tests to make sure it stays below 150. I take a lot of other vitamins and minerals and the nodules in my thyroid went from stage 4 to stage 3.

I went to see 2 doctors and they had different opinions: one said it is better to take 30.000 IU every 2 days for better absorption and the other one said it is better to have a stable level. I prefer a stable level.