PARATHYROID GLAND

endocrine gland occurring in all vertebrate species from amphibia upward, usually located close to and behind the thyroid gland and secreting parathormone, a hormone that regulates and maintains a normal calcium level in the blood serum. Human parathyroid tissue is brownish red, and the tiny glands may number as few as two and as many as eight on each side of the thyroid. Microscopically, they are made up of closely packed epithelial cells separated by thin fibrous bands; occasionally the cells are arranged in circles with an open centre (alveolar arrangement), which may contain a colloid material. The exact action of the internal secretion of these glands is not clear. It is believed that the parathyroid hormone performs an important role in stabilizing the calcium concentration of the body fluidsa very important function, since a lowering of the calcium-ion concentration results in a condition of increased excitability of nerves and muscles known as tetany, which results in muscular spasms, convulsions, and sometimes dementia. Under normal conditions a small drop in the calcium concentration of the body fluids results in increased activity of the parathyroid glands, which raises the calcium concentration by mobilizing some of the skeletal calcium. On the other hand, a rise in body-fluid calcium concentration above the normal is counteracted by a reduction or cessation of secretion of parathyroid hormone. The internal secretion of the parathyroids also affects the metabolism of phosphorus, an excess of the hormone resulting in a lowering of inorganic-phosphate concentration in the blood serum and an increased excretion of phosphate in the urine. Reduced parathyroid function is accompanied by a rise in the inorganic phosphate of the serum and a lessened urinary output of phosphate. Parathyroid hormone appears to play a role in the regulation of magnesium metabolism as well, possibly increasing the excretion of that electrolyte.