Extracellular calcium ion concentration is tightly regulated through the actions of parathyroid hormone (PTH) on kidney and bone. The intact peptide is secreted from the parathyroid glands at a rate that is appropriate to, and dependent upon the prevailing extracellular calcium ion concentration. Hypercalcemic or hypocalcemic disorders can be classified according to whether they arise from an excess or deficiency of PTH, a defect in the receptor for PTH (i.e., the PTH/PTHrP receptor), or insensitivity to PTH caused by defects downstream of the PTH/PTHrP receptor. The PTH gene is located on chromosome 11p15 and consists of three exons, which are separated by two introns. Exon 1 of the PTH gene is 85 bp in length and is untranslated whereas exons 2 and 3 encode the 115 amino acid pre-proPTH peptide.