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Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. It acts to increase the concentration of calcium (Ca2+) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells (C cells) of the thyroid gland) acts to decrease calcium concentration. PTH acts to increase the concentration of calcium in the blood by acting upon parathyroid hormone receptor in three parts of the body.
A high level of PTH in the blood is known as hyperparathyroidism.
If the cause is in the parathyroid gland it is called primary hyperparathyroidism. The causes are parathyroid adenoma, parathyroid hyperplasia and parathyroid cancer.
If the cause is outside the gland, it is known as secondary hyperparathyroidism. This can occur in chronic renal failure. In secondary hyperparathyroidism, serum Calcium levels are decreased, which causes the hypersecretion of PTH from the parathyroid glands. PTH acts on the proximal tubules in the kidney to decrease reabsorption of Phosphate (increasing its excretion in urine, decreasing its serum concentration). NOTE: however, in chronic renal failure, because the kidneys are failing they are unable to excrete phosphate in the urine, so in this case of secondary hyperparathyroidism, serum calcium will be decreased, but serum phosphate will be increased.
A low level of PTH in the blood is known as hypoparathyroidism. Causes include surgical misadventure (eg inadvertent removal during routine thyroid surgery), autoimmune disorder, and inborn errors of metabolism.