1. Endocrine Laboratory , Department of Clinical Chemistry , VU University Medical Center , Amsterdam , The Netherlands
2. Department of Nephrology , VU University Medical Center , Amsterdam , The Netherlands
3. Institute for Cardiovascular Research , VU University Medical Center , Amsterdam , The Netherlands
4. Laboratory of Endocrinology , Department of Clinical Chemistry , Academic Medical Center , Amsterdam , The Netherlands
5. Endocrine Section, Department of Internal Medicine , VU University Medical Center , Amsterdam , The Netherlands