PARATHYROID HORMONE IS A SINGLE-CHAIN POLYPEPTIDE ISOLATED FROM THE PARATHYROID GLANDS. IT CONTAINS 84 AMINO ACIDS AND IN MAN THE FIRST (N-TERMINAL) 34 APPEAR TO BE RESPONSIBLE FOR THE HORMONAL ACTIVITY. THE AMINO-ACID SEQUENCE VARIES ACCORDING TO THE SOURCE.
ENDOGENOUS PARATHYROID HORMONE IS INVOLVED IN THE MAINTENANCE OF PLASMA-CALCIUM CONCENTRATIONS THROUGH ITS ACTIONS ON BONE, KIDNEY, AND INDIRECTLY ON THE GASTROINTESTINAL TRACT.