PTH acts to increase the concentraion of calcium in the blood in three ways. It enhances the release of calcium from the large reservoir contained in the bones; it enhances reabsorption of calcium from renal tubules; and it enhances the absorption of calcium in the intestine.
PTH also acts to decrease the concentration of phosphate in the blood, primarily by redeucing reabsorption in the proximal tubules of the kidney.
Increased calcium concentration in the blood acts (via feedback inhibition) to decrease PTH secretion by the parathyroid glands.
Excessive PTH secretion is known as hyperparathyroidism, and is often the result of parathyroid tumor (primary hyperparathyroidism) or chronic renal failure (secondary hyperparathyroidism).
Insufficient PTH secretion is known as hypoparathyroidism, and is commonly caused by surgical misadventure, autoimmune disorder, or inborn errors of metabolism.
PTH can be measure in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.