1. Department of Medicine, Vagelos College of Physicians and Surgeons , New York, New York 10032 , USA
2. Department of Medicine, Geriatrics Faculty, Medical Sciences Center, University of Wisconsin , Madison, Wisconsin 53705 , USA
3. Department of Biochemistry, University of Wisconsin–Madison , Madison, Wisconsin 53706 , USA
4. Rheumatology Unit, Department of Medicine, University of Verona , 37134 Verona , Italy
5. Institute of Endocrine and Metabolic Sciences (IEMS), San Raffaele Vita–Salute University, IRCCS San Raffaele Hospital , 20132 Milan , Italy
6. Calcium Metabolism and Osteoporosis Program, WHO Collaborating Center for Metabolic Bone Disorders, American University of Beirut , Beirut , Lebanon
7. Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC, Vrije Universiteit Amsterdam , 1105AZ Amsterdam , the Netherlands
8. Amsterdam Gastroenterology, Endocrinology & Metabolism , 1105AZ Amsterdam , the Netherlands
9. Department of Clinical Chemistry, Endocrine Laboratory, Amsterdam UMC, University of Amsterdam , 1105AZ Amsterdam , the Netherlands