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The parathyroid glands are the tiny, round, yellow-brown that are nearly hidden from view in the posterior aspect of the thyroid gland. A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four of these glands. The number varies from one individual to another and therefore some may be located in other regions of the neck or chest. The parathyroid glandular cells are arranged in thick, branching rods containing scattered oxyphil cells and large numbers of smaller parathyroid cells.
The function of the oxyphil cells is unclear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH) or parathormone, the major hormone involved in the regulation of blood calcium levels. Controlling calcium level is critical because Ca2+ homeostasis is essential for many functions including transmission of nerve impulses, muscle contraction, and blood clotting. Falling blood calcium levels enhance PTH release and rising blood calcium level inhibit its PTH release. When the calcium level is low in blood, the PTH increases Ca2+ levels in blood by stimulating three target organs such as the skeleton, the kidneys, and the intestine. The PTH secretion causes the release of calcium from the bones by stimulating osteoclasts or resorption of bone, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH causes increased reabsorption of calcium in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol, also known as 1,25-dihydroxyvitamin D, which is the active form of vitamin D3, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.
Abnormally high activity of the parathyroid gland can cause hyperparathyroidism, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. It can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced, which leads to abnormal reflexes and weak skeletal muscles. At the same time, calcium deposits may collect in the body’s tissues and organs, impairing their functioning.
Hypoparathyroidism most often follows parathyroid gland trauma or removal during thyroid surgery. However, extending deficiency of dietary magnesium also can cause functional hypoparathyroidism. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyze muscles, including those involved in breathing, and can be fatal.
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