Elevated fibroblast growth factor 23 levels in a newborn with secondary hypoparathyroidism.

Fibroblast growth factor 23 (FGF-23) is a recently identified hormone that is of prime importance for phosphate homeostasis in humans. FGF-23 is secreted by osteocytes in response to phosphate-loading. It stimulates renal phosphate excretion and suppresses the formation of 1.25-dihydroxy-vitamin D by inhibiting renal 1α-hydroxylase activity. Knowledge about FGF-23 in early infancy is limited. We report here the case of a newborn with transient secondary hypoparathyroidism caused by maternal primary hyperparathyroidism during pregnancy. FGF-23 levels at birth were extremely high in the child (15.850 kilo-Relative Units per liter, kRU/L) (ie, ∼45 times higher than in the mother) and ∼7 times higher than in healthy newborns. The child's FGF-23 levels declined gradually and reached the normal adult range after ∼7 months. We discuss the potential physiologic significance of FGF-23 in newborns.