 Subjects will be screened on the MGH General Clinical Research Center GCRC. After obtaining informed consent for the screening procedures, subjects will undergo a complete history and physical examination. If no exclusionary findings are noted during the history and physical examination, blood will be drawn to measure hemoglobin, routine chemistries including serum calcium, liver function tests, and creatinine, and serum levels of PTH, 25-OH vitamin D, TSH, T, and PSA. 