Late-onset neonatal hypocalcemia is due to relative resistance of the immature kidney to which hormone?

• A. Calcium
• B. Parathyroid hormone
• C. Magnesium
• D. Vitamin D

Answer: (B)

Late-onset neonatal hypocalcemia occurs because the newborn kidney is not fully responsive to parathyroid hormone. Parathyroid hormone normally raises serum calcium by enhancing calcium reabsorption in the kidney’s distal tubule and by stimulating 1-alpha-hydroxylase to activate vitamin D, which increases intestinal calcium absorption. In the immature neonatal kidney, this response is blunted, so calcium reabsorption is reduced and vitamin D activation is limited. When dietary calcium from breast milk is relatively low and stores are diminishing after the first week, serum calcium falls despite parathyroid hormone being present or even elevated as the body attempts to compensate. This relative resistance of the kidney to parathyroid hormone is the driving factor behind late-onset hypocalcemia.