Endocrine Glands

Parathyroid Glands

Most people have four parathyroid glands; however, the number can vary from two to six. These glands are located on the posterior surface of the thyroid gland, as shown in Figure. Normally, there is a superior gland and an inferior gland associated with each of the thyroid’s two lobes. Each parathyroid gland is covered by connective tissue and contains many secretory cells that are associated with a capillary network.

The parathyroid glands are round structures located on the surface of the right and left lobes of the thyroid gland. In the illustration shown, there are two parathyroid glands on each side, and one is located above the other.
The parathyroid glands are located on the posterior of the thyroid gland. (credit: modification of work by NCI)

The parathyroid glands produce parathyroid hormone (PTH). PTH increases blood calcium concentrations when calcium ion levels fall below normal. PTH (1) enhances reabsorption of Ca2+ by the kidneys, (2) stimulates osteoclast activity and inhibits osteoblast activity, and (3) it stimulates synthesis and secretion of calcitriol by the kidneys, which enhances Ca2+ absorption by the digestive system. PTH is produced by chief cells of the parathyroid. PTH and calcitonin work in opposition to one another to maintain homeostatic Ca2+ levels in body fluids. Another type of cells, oxyphil cells, exist in the parathyroid but their function is not known. These hormones encourage bone growth, muscle mass, and blood cell formation in children and women.