Hormones

The parathyroid glands are attached to the bottom of the thyroid gland. They secrete the polypeptide parathyroid hormone (PTH) which plays a crucial role in monitoring blood calcium and phosphate levels. About 99% of the body's calcium is in the bones, and 85% of the magnesium is also found in bone. Low blood levels of calcium stimulate PTH release into the bloodstream in two steps. Initially, calcium is released from the fluid around bone cells. And later, calcium can be drawn from bone itself. Although, only about 1% of bone calcium is readily exchangeable. PTH can also increase the absorption of calcium in the intestines by stimulating the kidneys to produce a vitamin D-like substance which facilitates this action. High blood calcium levels will inhibit PTH action, and magnesium (which is chemically similar to calcium) shows a similar effect.

Calcium is a critical element for the human body. Even though the majority of calcium is in bone, it is also used by muscles, including cardiac muscle for contractions, and by nerves in the release of neurotransmitters. Calcium is a powerful messenger in the immune response of inflammation and blood clotting. Both PTH and calcitonin regulate calcium levels in the kidneys, the gut, bone, and blood. Whereas calcitonin is released in conditions of high blood calcium levels, PTH is released when calcium levels fall in the blood. Comparing the two, PTH causes an increase in calcium absorption in the kidneys, absorption in the intestine, release from bone, and levels in the blood. In addition, PTH decreases kidney phosphate absorption. Calcitonin has the opposite effect on each of these variables. PTH is thought to be the major calcium modulator in adults.

PTH deficiency can be due to autoimmune diseases or to inherited parathyroid gland problems. Low PTH capabilities cause depressed blood calcium levels and neuromuscular problems. Very low PTH can lead to tetany or muscle spasms. Excess PTH can lead to weakened bones because it causes too much calcium to be drawn from the bones and to be excreted in the urine. Abnormalities of bone mineral deposits can lead to a number of conditions including osteoporosis and rickets. Osteoporosis can be due to dietary insufficiencies of calcium, phosphate, or vitamin C (which has an important role in formation of the bone matrix). The end result is a loss of bone mass. Rickets is usually caused by a vitamin D deficiency and results in lower rates of bone matrix formation in children. These examples show how important a balanced nutritious diet is for healthy development.