(See also Overview of the Thyroid Gland; Hypercalcemia; and Hypocalcemia.)
The thyroid is a small gland, measuring about 2 inches (5 centimeters) across, that lies just under the skin below the Adam’s apple in the neck. The thyroid gland secretes thyroid hormones, which control the speed at which the body’s chemical functions (also called metabolism) proceed. The parathyroid glands are so named because they are located near the thyroid gland. There are usually four pea-sized parathyroid glands, one located behind each corner of the thyroid, but their exact location and even the total number of glands are quite variable.
The parathyroid glands secrete parathyroid hormone (PTH), which regulates calcium levels in the blood and tissues through its effects on bones, the kidneys, and the intestine. PTH raises the level of calcium in the blood when the level of calcium is too low (a condition called hypocalcemia). The calcium level may fall as a result of any of the following:
A parathyroid gland disorder causing a low parathyroid hormone level (hypoparathyroidism)
Too little calcium in the diet
A kidney disorder
Certain medications
If not corrected, hypocalcemia can progress to cause muscle cramps, confusion, depression, forgetfulness, and tingling in the lips, fingers, and feet. Stiff, achy muscles also may result. Severely decreased levels of calcium in the blood can cause muscle spasms, seizures, and abnormal heart rhythms.
The Parathyroid Glands
Parathyroid cells sense when the level of calcium in the blood is low and then release PTH into the bloodstream. Within minutes, PTH increases calcium in the blood by doing the following:
Rapidly release the calcium and phosphate that is stored in bones (bone resorption)
Help the kidneys conserve calcium
Increase the ability of the intestines to absorb calcium from the diet
PTH enhances calcium reabsorption from blood filtered in the kidneys (see also Kidneys). PTH also decreases the amount of the phosphate that the kidneys reabsorb, which increases the amount of phosphate lost in the urine.
PTH also stimulates conversion of vitamin D to its most active form, calcitriol. This conversion also helps raise levels of calcium in the blood because calcitriol increases how much calcium is absorbed from the diet by the intestine. Both PTH and vitamin D help regulate bone growth and bone remodeling (see also ).
However, long-term increases in PTH result in too much bone resorption. During the process of bone resorption, specialized bone cells involved in the growth and healing of bone (osteoclasts) break down the tissue in bones and release its minerals into the blood. This results in an increase in the amount of calcium transferred from bone tissue to the blood and can eventually lead to osteoporosis and other diseases associated with bone loss.
Doctors usually check a person's PTH level by doing a specific type of blood test called a radioimmunoassay.
