Parathyroid hormone (PTH) is an 84-amino acid peptide hormone secreted by the parathyroid gland in response to low levels of calcium in the blood. Parathyroid hormone (PTH) acts to maintain normal levels of blood calcium by:
increasing the number of osteoclasts, cells that break down the bone matrix and release calcium into the blood
increasing the reabsorption of calcium and magnesium ions in the kidney tubules, so that their concentration is maintained in the blood
converting vitamin D to its active form, which increases calcium absorption in the intestine.
Parathyroid hormone acts in opposition to calcitonin.