Biological or Clinical Significance:
Parathyroid hormone (parathyrin, PTH), a single-chain polypeptide containing 84 amino acids, is synthesized and secreted by the parathyroid glands. PTH undergoes proteolysis in the liver but also in the kidneys and bone to yield N-terminal fragments (1-37 amino acids) and C-terminal and mid-region fragments (38-84 amino acids). The biologically active N-terminal fragment has a half-life of only 2 to 5 minutes, whereas the C-terminal fragment has a half-life of several hours. PTH exerts significant influence in the maintenance of optimal calcium ion concentrations. PTH raises serum ionized calcium levels through direct action on bone and the kidneys: It increases the rate of calcium ion flow from bone to the extracellular fluid, and increases both the renal tubular reabsorption of ionized calcium and the renal excretion of phosphate. Long-term regulation of total body calcium by PTH occurs through its stimulation of vitamin D metabolism, which results in enhanced intestinal absorption of ionized calcium. Since PTH maintains calcium homeostasis, its measurement is an important aid in the diagnosis of calcium metabolism disorders. Decreased serum calcium levels result in increased PTH secretion, causing increased absorption of dietary calcium, decreased renal clearance and mobilization of skeletal calcium stores. In conjunction with calcium levels, PTH quantification can help distinguish between normal patients and patients with hyperparathyroidism, hypoparathyroidism or hypercalcemia of malignancy.
Principle of Test Method:
Whole PTH is an immunoradiometric (IRMA) assay utilizing polyclonal PTH antibodies directed against N-terminal PTH and C-terminal PTH. The use of these antibodies guarantees that only Whole PTH (amino acids 1-84) is detected.