WHAT IS PARATHYROID HORMONE (PTH) TEST?
Parathyroid hormone (PTH) is measured by radioassay using competitive protein-binding agents to identify and measure several molecular forms of PTH: intact and midmolecule fragments. N-terminal fragments and C-terminal fragments may be tested in some laboratories. Intact PTH is secreted by the parathyroid gland and is metabolized by the liver and kidneys into N-terminal fragments and C-terminal fragments. The intact and N-terminal fragments are helpful in identifying acute conditions, whereas C-terminal fragments indicate chronic disturbances of PTH metabolism. Parathyroid hormone is directly responsible for the plasma regulation of calcium and phosphorus. When the body’s normal autoregulatory mechanism senses a decrease in serum calcium level, the parathyroid gland is stimulated to secrete PTH. The elevated serum PTH triggers the release of calcium from bone and stimulates the renal tubules to increase reabsorption of calcium ions in the distal convoluted tubules and to decrease reabsorption of phosphorus in the proximal convoluted tubules. When the serum calcium concentration again becomes adequate, the parathyroid gland decreases PTH secretion. In the presence of primary parathyroid tumour or hyperplasia, the PTH-calcium autoregulation fails. As PTH secretion increases, so does the serum calcium level, ultimately resulting in a hypercalcemic condition that can be life-threatening. Assessment of radioassayed PTH is helpful in differentiating parathyroid causes from nonparathyroid causes of hypercalcemia. Other causes of hypercalcemia generally display normal to slightly high or low PTH secretion. Thus, PTH should always be evaluated in conjunction with serum ionized calcium levels.