PTH, or parathyroid hormone, is produced by four small glands, called the parathyroid glands, which are located in your neck behind your thyroid gland. Parathyroid hormone works with calcitonin, a hormone made by the thyroid gland, to control calcium homeostasis, which is the regulation of calcium ions in the fluids outside of your cells, such as your blood.
Physiology
Most of the calcium in your body, approximately 99 percent, is located inside of your bones. The remaining calcium circulates throughout your body in your extracellular fluids, such as your blood. The concentration of blood calcium is so low, 8.5 to 10.2 mg/dL, that even minor changes in calcium concentration can cause serious health problems. When your blood calcium levels drop, your parathyroid glands release PTH to help raise blood calcium levels back to normal. Once your blood calcium levels return to normal, your parathyroid glands stop releasing PTH. PTH maintains calcium homeostasis through three different physiological processes.
PTH and Bone
Osteoblasts are the cells responsible for producing new bone. When your parathyroid glands release PTH, the hormone stimulates these osteoblasts to release calcium ions out of the bone and into your blood. The release of calcium ions into the blood increases blood calcium levels. Once levels return to normal, PTH production decreases and osteoblasts stop releasing calcium.
PTH and Kidneys
Parathyroid hormone also has an effect on the kidneys. PTH prevents the loss of calcium in the urine by stimulating calcium reabsorption. Instead of removing calcium from your body through the urine, the kidneys leave calcium in your blood in an effort to restore normal blood calcium levels.
PTH and Small Intestine
Parathyroid hormone also stimulates your small intestine to absorb more calcium, although this process is indirect. PTH triggers your kidneys to increase production of the active form of vitamin D. Vitamin D allows your small intestine to absorb more calcium from the foods you eat, thus increasing the levels of calcium in your blood.
Considerations
PTH is made up of 84 different amino acids and has a very short life span. As soon as PTH enters the blood, it begins to break down into fragments, and levels drop by half in about five minutes, according to Lab Tests Online. These fragments remain active, but they have the opposite effect of PTH as a whole. Instead of increasing blood calcium levels, these fragments decrease blood calcium levels and stop the release of calcium from the bones.
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