Parathyroid hormone (1 - 13) (human) is amino acids 1-13 of the N-terminus of parathyroid hormone and is a tryptic fragment sharing only 6 and 7 amino acids with the N-terminal tryptic peptides of parathyroid hormone related protein and parathyroid hormone (7-84) respectively. Parathyroid hormone (1 - 13) (human) has no substantial homology with any other trypsin digestion fragments of human proteins, making it a highly specific marker for full length parathyroid hormone (1-84).