For the past 4 years I have been taking about
3,000 IU of D3 daily and can bearly maintain optimum blood levels. I find that it is easier to keep my levels up in the summer with all the sunshine than in winter.
But Vitamin D is one of the few vitamins that it is possible to OD on. It costs about an extra $30 to include a test for D during your regular blood work or checking cholesterol levels. After testing most people find that they will need more D than expected, but it is possible that some may take too much D and that can cause problems.
Optimum blood levels are around 40 to 65 ng/ml (100 to 160 nmol). Quest Diagnostics needs to clean up their procedures for measuring Vitamin D. Quest's numbers are usually too high. For example a reading of 40 from LabCorp, ARUP, or Clinical Pathology will show up in a Quest reading of about 48 or so.
Google 'vitamin d cancer', heart disease, crohn's, ulcerative colitis, or anything. It is pretty much a panacea vitamin. I find that my blood levels drop when I was flaring so I guess my body was using it.