By Elaine Moore
Hormones and Health
Hypoparathyroidism is characterized by decreased blood levels of parathyroid hormone. Parathyroid hormone regulates calcium in cells throughout the body. In hypoparathyroidism, the subject of this article, levels of parathyroid hormone are deficient.
Homeostasis and Reference Ranges
Parathyroid hormone (PTH), along with vitamin D & calcitonin, work to regulate the body’s calcium levels.
From this average or mean, a reference or normal range is established. In calculating the reference range for a particular substance, 50% of the normal subjects in the study fall below the mean and 50% of the subjects fall above the mean. These normal or reference ranges serve as guidelines which aid in the diagnosis and treatment of disease. While hypoparathyroidism is diagnosed by laboratory measurements of parathyroid hormone with levels of PTH generally being low or undetectable, a low blood calcium level is often the first clue that something is wrong. In addition, serum phosphorus levels are increased in hypoparathyroidism. Phosphorus, like calcium, is involved in bone formation and resorption. Consequently, PTH also influences phosphorus excretion.
The Normal Function of the Parathyroid Gland
Ions such as calcium and potassium travel inside and outside of cells as needed. Through a system of membrane channels and ion pumps, ions are stored in blood cells and released into the extracellular circulation as needed. Extracellular is a term that refers to the amount of calcium in the liquid portion of blood, rather than in the blood cells.
The primary function of the parathyroid gland is to regulate calcium levels in the body. All of the body’s tissues, including blood, are composed of several different types of cells, all with specific functions. For instance, the tissue that comprises the parathyroid gland is made up several different cell types. The most functionally important are the “chief” cells, which produce and store parathyroid hormone.
Each of the parathyroid gland’s “chief” cells contains a sensor or protein receptor attached to its outer surface. This sensor tells the cell how much extracellular calcium is present. If the sensors detect that there is too much or too little extracellular calcium, they order the parathyroid glands to, respectively, cut back or increase the rate of parathyroid hormone released from the parathyroid glands.
Half of the body’s extracellular supply of calcium is linked or bound to protein molecules and unable to interact with other cells. The other half is ionized or free calcium. Calcium is required for the proper functioning of many tissues, including excitation-contraction coupling in the heart and other muscles, proper transmission of impulses within the nervous system, platelet aggregation, blood coagulation, cell reproduction, and the synthesis of other hormones.
Calcium Metabolism and PTH Secretion
Calcium metabolism is remarkable in that the body’s stores of ionized calcium, which represent a small fraction of the body’s total calcium stores, are kept within a tightly regulated range despite the rapid changes in calcium utilization that are constantly occurring. When sensors in the parathyroid cells detect increased levels of ionized calcium in the blood, they order the chief cells to stop releasing PTH.
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