Parathyroid cysts: Functional and mediastinal.

: A report of a functioning mediastinal parathyroid cyst is presented with a review of the world literature. This represents the tenth functional, eleventh mediastinal, and third functioning mediastinal parathyroid cyst reported. The predilection of functional cysts for aberrant locations would suggest on hypothesis of their occurrence by degeneration of pre-existing adenomas based on an inadequacy of their blood supply. The criteria for ascribing functional activity to a parathyroid cyst are presented and discussed.