The parathyroids are ovoid glands measuring 6 mm in their greatest diameter. There are two pairs, superior and inferior, derived from the fourth and third branchial arches respectively. They lie within the fascial capsule of the thyroid, intimately related to its posterior border. Ectopic parathyroid glands (particularly the inferior ones) are not uncommon, a frequent site being the superior mediastinum. Parathyroid hormone (PTH) is secreted in response to falling plasma calcium concentrations. By a variety of mechanisms (including increased renal tubular reabsorption, increased resorption from bone and increased synthesis/activity of vitamin D) this restores serum calcium towards normal.
