Parathyroid hormone
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Parathyroid hormone (PTH) is secreted by the parathyroid glands as a polypeptide containing 84 amino acids.
PTH acts to increase the concentration of calcium in the blood in three ways. It enhances the release of calcium from the large reservoir contained in the bones; it enhances reabsorption of calcium from renal tubules; and it enhances the absorption of calcium in the intestine by increasing the production of 1,25-DiHydroxy vitamin D3.
PTH also acts to decrease the concentration of phosphate in the blood, primarily by reducing reabsorption in the proximal tubules of the kidney.
Increased calcium concentration in the blood acts (via feedback inhibition) to decrease PTH secretion by the parathyroid glands. This is achieved by the activation of calcium-sensing receptors located on parathyroid cells.
Excessive PTH secretion is known as hyperparathyroidism, and is often the result of a benign parathyroid tumor (primary hyperparathyroidism) that loses its sensitivity to circulating calcium levels. In chronic renal failure secondary hyperparathyroidism can result.
Insufficient PTH secretion is known as hypoparathyroidism, and is commonly caused by surgical misadventure, autoimmune disorder, or inborn errors of metabolism.
PTH can be measured in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.