"It is projected that raising the minimum year-around serum 25(OH)D level to 40 to 60 ng/mL (100-150 nmol/L) would prevent approximately 58,000 new cases of breast cancer and 49,000 new cases of colorectal cancer each year, and three fourths of deaths from these diseases in the United States and Canada, based on observational studies combined with a randomized trial."
Vitamin D for cancer prevention: global perspective.
Garland CF, et al. Ann Epidemiol. 2009.
If you will click on my name and then scroll down to my Journal you will find a one page Overview of a paper on Diagnosis and Treatment of Hypothyroidism: A Patient's Perspective. If you go to that link and go to Rec. 14 on page 14 you will find that the Vitamin D council recommends a minimum of 50 ng/mL.