Serum 25-hydroxy Vitamin D [25(OH)D] Levels, Supplemental Vitamin D, and Parathyroid Hormone Levels in Premature Infants

This study will determine levels of vitamin D supplementation to achieve goal serum 25-hydroxy vitamin D \[25(OH)D\] levels of 30 ng/mL, and to define serum 25(OH)D levels required to achieve suppression of parathyroid hormone in preterm newborn infants hospitalized in Newborn Intensive Care Nursery (NICU). Infants 23 weeks gestational age or greater will be randomized to two different levels of vitamin D supplementation: 400 IU vitamin D3/day, or 800 IU vitamin D3/day.