Parathyroid Gland

The parathyroid glands are small endocrine glands attached to the thyroid. In humans there are four parathyroid glands, located on the back (dorsal) side of the thyroid gland — one pair embedded in each of the two thyroid lobes.

Hormone secreted

The parathyroid glands secrete a peptide hormone known as parathyroid hormone (PTH). The amount of PTH released is controlled by the level of calcium ions circulating in the blood: the glands respond to how much calcium is present and adjust their secretion accordingly.

Action of PTH

Parathyroid hormone raises the level of calcium ions in the blood. It does this by acting on three targets:

• Bone: PTH acts on bones and stimulates bone resorption — the dissolution or demineralisation of bone — which releases calcium into the blood.
• Kidney: it stimulates the renal tubules to reabsorb calcium ions, so that less calcium is lost in the urine.
• Gut: it increases the absorption of calcium from digested food in the intestine.

Because all three actions push blood calcium upward, PTH is described as a hypercalcemic hormone — one that raises blood calcium levels.

Working with calcitonin …