After doing a little reading, that dosage seems perfectly reasonable and safe. In the actual scientific literature there isnt evidence of hypercalcemia in adults until around 50,000+IU (too much vitamin D makes you hold onto calcium in your blood), and even then only after several months of that dose. 10-12K appears to be well with the safe range, as there doesnt appear to be any evidence that harm can occur from such a dose, despite people regularly taking this amount. The NIH recommended max of 4K per day seems based on an assumption that it should be sufficient to get you in the recommended range (NIH assumptions are always questionable), rather than there being any actual evidence of harms from doses exceeding the recommendation.

u/elchupacabra