There are two pairs of parathyroid glands on the posterior aspect of the thyroid gland. The parathyroid gland is responsible for the production of parathormone.
The parathyroid glands contain specialized cells called chief cells, which monitor the concentration of serum calcium. If the calcium concentration drops below normal, parathormone is secreted. Parathormone acts on the bone cells by stimulating osteoclasts (bone-destroying cells) and inhibiting osteoblasts (bone-producing cells), thus releasing calcium from the bone into the bloodstream. It also increases calcium uptake from the intestine, and reduces urinary excretion of calcium.