While I agree with the general point

Vitamin D is toxic (and ultimately fatal) at high doses, which is why the 'suggested' dosages of between 400IU and 1000IU are so conservative. You may need more, but you should get a blood test.

THIS. And not just for Vitamin D. Not everyone absorbs / metabolizes / excretes vitamins or minerals in the same way. Learning whether or not you're an outlier can be done either the safe way, or the dangerous way.