Parathyroid Glands

There are four parathyroid glands and are located on the posterior portion of the thyroid gland. They monitor the levels of calcium in the blood and secrete parathyroid hormone (PTH) in response to low calcium levels. PTH also has an effect on phosphorus and vitamin D levels discussed below.

Unlike the thyroid the parathyroid glands are not controlled by the pituitary or hypothalamus. They monitor calcium levels directly and release or restrict PTH as needed. Phosphorus levels are also monitored and can effect PTH levels.

Actions of PTH
The main function of PTH is to regulate calcium levels. It does this in three ways. First is to increase resorbtion (release) of calcium from the bone into the circulatory system (this also inhibits bone formation). Second is to increase calcium reabsorption (decrease excretion) in the kidneys. Third is to convert vitamin D in the intestines to the more active form of D3 which enhances intestinal uptake of calcium.

When calcium is resorbed (released) from the bone so is phosphorus. In response to increased PTH levels the kidneys increase the excretion of phosphorus.