Parathyroid gland

Humans have four tiny parathyroid glands, embedded in the posterior wall of the thyroid gland. Each gland is built from two cell types: chief cells, which secrete parathyroid hormone (PTH), and oxyphil cells, whose function is not yet known.

Parathyroid hormone (PTH, or parathormone) is a peptide, hypercalcemic hormone — meaning its overall effect is to raise blood calcium. Its own secretion is controlled directly by the blood calcium level (falling calcium triggers more PTH release). PTH raises blood calcium by stimulating bone-resorbing cells called osteoclasts to dissolve the mineral matrix of bone, releasing both calcium and phosphate into the blood; it also enhances calcium reabsorption and phosphate excretion by the renal tubules of the kidney, and promotes activation of vitamin D, which in turn increases calcium absorption from t …