Parathyroid hormone
![helical dimer structure of hPTH-(1-34)[5]](/uploads/202502/01/The_ribbon_cartoon_structure2709.png)
![A diagrammatic representation of the movements of calcium ions into and out of the blood plasma (the central square labeled PLASMA Ca2+) in an adult in calcium balance. The widths of the red arrows indicating movement into and out of the plasma are roughly in proportion to the daily amounts of calcium moved in the indicated directions. The size of the central square in not in proportion to the size of the diagrammatic bone, which represents the calcium present in the skeleton, and contains approximately 25,000 mmol (or 1 kg) of calcium compared to the 9 mmol (360 mg) dissolved in the blood plasma. The differently colored narrow arrows indicate where the specified hormones act, and their effects (“+” means stimulates; “-“ means inhibits) when their plasma levels are high. PTH is parathyroid hormone, 1,25 OH VIT D3 is calcitriol or 1,25 dihydroxyvitamin D3, and CALCITONIN is a hormone secreted by the thyroid gland when the plasma ionized calcium level is high or rising. The diagram does not show the extremely small amounts of calcium that move into and out of the cells of the body, nor does it indicate the calcium that is bound to the extracellular proteins (in particular the plasma proteins) or to plasma phosphate.[6][7][8][9][10]](/uploads/202502/01/Calcium_balance_22709.jpg)
Parathyroid hormone (PTH), parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids, yet effective hormone-receptor interaction requires solely the 34-N-terminal amino acids. While PTH acts to increase the concentration of ionic calcium (Ca) in the blood, calcitonin, a hormone produced by the parafollicular cells (C cells) of the thyroid gland, acts to decrease ionic calcium concentration. PTH essentially acts to increase the concentration of calcium in the blood by acting upon the parathyroid hormone 1 receptor, which is present at high levels in bone and kidney, and the parathyroid hormone 2 receptor, which is present at high levels in the central nervous system, pancreas, testis, and placenta. PTH half-life is approximately 4 minutes. It has a molecular mass of approximately 9500 Da.