Many of us take vitamin D and for good reasons. This study has identified a risk of overdoing it. Look at the numbers. A serum Vit. D test is simple (25-hydroxy Vitamin D test) and for those of us living in temperate zones with little sun exposure in winter and lots of sun in summer it is a good idea to vary Vit. D3 supplementation according to how much sun you are getting so as not to go out of the "Goldilocks" zone. I get tested after the end of the 2nd darkest month and in the middle of summer.