Two hormones hold blood calcium steady.

Blood Ca2+ is held in range by two opposing hormones: low Ca2+ triggers PTH (bone resorption, kidney reabsorption, gut absorption), high Ca2+ triggers thyrocalcitonin.

Keep blood Ca2+ between 9 and 11 mg/dL for a week on a 0.2 g/day diet, then remove the parathyroids and see which way it drifts.

PTH lowers blood calcium; TCT raises it.

• 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.

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