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The thyroid gland's posterior surface contains two embedded pairs of parathyroid glands. They are separated from the thyroid gland by a dense capsule.

The parathyroid glands primarily have two cell populations, namely oxyphil and principal parathyroid cells.

The oxyphil cells appear in late puberty, but their functions currently are unknown.

The principal parathyroid cells produce the parathyroid hormone, PTH or parathormone.

These cells monitor the circulating calcium concentrations.

When the calcium levels drop, the cells secrete the PTH. This hormone affects osteoblast and osteoclast activity in the bones to mobilize calcium.

The increased blood calcium levels enhance reuptake at the kidney and calcitriol secretion, enhancing the PTH activity.