Unusual case of neonatal hypercalcemia.

We present a rare case of neonatal hyperparathyroidism secondary to a homozygous calcium sensing receptor (CASR) mutation, diagnosed by the genetics team. The CASR mutation is a homozygous inactivating mutation at the calcium sensing receptor. Inactivation of the receptor leads to hypercalcemia and activation leads to hypocalcemia. Heterozygous mutations can cause mild forms of asymptomatic hypercalcemia that often run in families. The homozygous mutation causes a rare form of neonatal severe hyperparathyroidism.