PTH and Hypocalcemia: Familial isolated hypocalcemia due to insufficient PTH secretion can be caused by several genetic alterations, including activating mutations in the calcium-sensing receptor (CaSR) (8), homozygous or heterozygous mutations in glial cells missing-2 (GCM2) (9, 10), heterozygous mutations in TBX1 (11), or activating Gα11 mutations (GNA11) that enhance signaling down-stream of the CaSR (12, 13).