What are clinical correlations w/parathyroid embryology?

-Accessory or supernumerary parathyroid glands are found in approximately 13% of individuals at autopsy -These glands most likely result from tissue fragmentation that occurs during the migration -The former typically weigh less than 50 mg, while true supernumerary glands have an average weight of 24 mg -Absence of parathyroids (ie, < 4 glands) is noted in approximately 3% of individuals at autopsy Ectopic parathyroid glands occur in 15-20% of patients -The glands may be located anywhere near or even within the thyroid or thymus -For example, if parathyroid III do not descend entirely, they may be located as high as the bifurcation of the common carotid artery -Conversely, if parathyroid III do not release from the thymus, they may be located intrathoracically, as low as the aortopulmonary window -Other common ectopic locations include the anterior mediastinum, posterior mediastinum, and retroesophageal and prevertebral regions