Vitamin D3 is actually a hormone. It is a naturally occuring substance in the body. You could produce tens of thousands of IU with a day at the beach. The advice here is good! You can safely take 4 - 6 thousand IU daily and not worry about
any sort of toxicity levels. You can also expose as much of your skin to the sun daily for 15 to 20 minutes or just before your skin starts to turn red. That time is slightly different for everyone.
Because D3 is a naturally occuring hormone in the body, there is very little risk of toxicity. For most people, (even the healthy ones) they are well below what they should have. D3 is also called the "antibiotic" vitiamin, as it is a strong immuno booster.
By they way, this is all stuff I have researched/read or heard from doctors; by all means talk to your doctor before adding anything to your regimen.