The causes of hypercalcemia during the neonatal period are varied. Diagnosis is based on the plasma levels of parathyroid hormone (PTH). (a) PTH in plasma is diminished; exogenous o r endogenous calcium overloading is suspected. (b) PTH in plasma is increased; hyperparathyroidism caused by homozygous or heterozygousloss-of-function mutations in the CaR gene is suspected. (c) PTH i n plasma is normal; low or normal urinary calcium excretion (i.e.relative hypocalciuria) and normal PTH plasma levelsare relevant findings of familial hypocalciuric hypercalcemia (FHH) syndrome.