PTH

PTH is produced by four parathyroid glands that are located in the neck behind the thyroid gland. Normally, these glands secrete PTH into the bloodstream in response to low blood calcium levels. Parathyroid hormone then works in three ways to help raise blood calcium levels back to normal. It takes calcium from the body's bone, stimulates the activation of vitamin D in the kidney (which in turn increases the absorption of calcium from the intestines), and suppresses the excretion of calcium in the urine (while encouraging excretion of phosphate). As calcium levels begin to increase in the blood, PTH normally decreases.

Parathyroid hormone itself is composed of 84 amino acids (sometimes called PTH (1-84)). Once it is released from the parathyroid gland into the blood stream, it has a very short life-span; levels fall by half in less than 5 minutes. The fall is caused primarily by the breakdown of PTH to smaller fragments, mainly PTH (35-84) and PTH (7-84). Although it was originally thought that these fragments are inactive, they are, in fact, active but have different actions than PTH (1-84). These fragments, (especially PTH (7-84)), seem to do the opposite of PTH: they lower serum calcium and prevent removal of calcium from the bone.