(See also Overview of the Thyroid Gland Overview of the Thyroid Gland 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 two halves (lobes) of the gland are connected... read more ; Hypercalcemia Hypercalcemia (High Level of Calcium in the Blood) In hypercalcemia, the level of calcium in blood is too high. A high calcium level may result from a problem with the parathyroid glands, as well as from diet, cancer, or disorders affecting... read more 
; and Hypocalcemia Hypocalcemia (Low Level of Calcium in the Blood) In hypocalcemia, the calcium level in blood is too low. A low calcium level may result from a problem with the parathyroid glands, as well as from diet, kidney disorders, or certain medications... read more .)
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) Hypocalcemia (Low Level of Calcium in the Blood) In hypocalcemia, the calcium level in blood is too low. A low calcium level may result from a problem with the parathyroid glands, as well as from diet, kidney disorders, or certain medications... read more . The calcium level may fall as a result of any of the following:
A parathyroid gland disorder causing a low parathyroid hormone level (hypoparathyroidism Hypoparathyroidism Hypoparathyroidism is a deficiency of parathyroid hormone (PTH) often caused by an autoimmune disorder, treatment-related damage to the parathyroid glands, or removal of the glands during surgery... read more )
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
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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 Overview of Phosphate's Role in the Body Phosphorus is an element that plays an important role in the body. In the body, almost all phosphorus is combined with oxygen, forming phosphate. Phosphate is one of the body's electrolytes... read more 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 Kidneys The kidneys are bean-shaped organs that figure prominently in the urinary tract. Each is about 4 to 5 inches (12 centimeters) long and weighs about one third of a pound (150 grams). One lies... read more ). 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 Bones 
(see also Vitamin D Deficiency Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more ).
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 Overview of Acid-Base Balance An important part of being healthy is for the blood to maintain a normal degree of acidity or alkalinity. The acidity or alkalinity of any solution, including blood, is indicated on the pH scale... read more 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.
