Parathyroid gland and thymus

Humans have four parathyroid glands on the back of the thyroid, one pair in each of its two lobes.

They secrete a peptide hormone, parathyroid hormone (PTH). How much PTH is secreted is controlled by the level of calcium ions circulating in the blood.

PTH raises blood Ca²⁺ in three ways: it stimulates bone resorption (dissolution or demineralisation of bone), it stimulates reabsorption of Ca²⁺ by the renal tubules, and it increases Ca²⁺ absorption from digested food. PTH is therefore a hypercalcemic hormone.

PTH and TCT together play a major part in keeping the body's calcium in balance.

The thymus is a lobular gland between the lungs, behind the sternum, on the ventral side of the aorta. It has a major role in developing the immune system.

It secretes peptide hormones called thymosins, which have a major role in the differentiation of T-lymphocytes, the cells that give cell-mediated immunity. Thymosins also promote antibody production, giving humoral immunity.

The thymus degenerates in old people, so less thymosin is made and their immune responses become weak.