Location of Parathyroid Glands

Abstract

The superior parathyroid glands are derived from the fourth branchial pouch and attach to the posterior border of the descending thyroid. The inferior tips of the pyriform sinuses are also derived from the fourth branchial pouch, and superior parathyroid glands may remain attached to the posterior wall of the pyriform sinuses, resulting in a retropharyngeal location. The inferior parathyroid glands are derived from the third pharyngeal pouch, along with the thymus, and are more variable in position than the upper glands. They usually descend with the thymus and usually detach from the thymus, ending up along or adjacent to the inferior margin or inferolateral margin of the lower pole of the thyroid. The inferior glands may descend lower, following the thymus, and lie in the thyrothymic ligament. The vascular pedicle of the inferior glands lies medial and anterior to the recurrent laryngeal nerve while the superior parathyroid glands are posterolateral to the nerve. For glands adjacent to the tracheo-esophageal groove, the gland may be anterior or posterior to the nerve. The nerve is not imaged on CT or ultrasound and is only defined at surgery. Completely intrathyroidal parathyroid adenomas, which can be derived from upper or lower parathyroid glands, are rare. Undescended parathyroid adenomas follow the course of thymic descent and lie close to the carotid. Anterior mediastinal parathyroid glands derive from inferior glands that follow the course of the thymus descent. Much rarer ectopic glands in the aortopulmonary (AP) window, middle mediastinum, and posterior mediastinum derive from superior parathyroid glands and have no adjacent thymic tissue. Enlarged ectopic mediastinal parathyroid glands are frequently supernumerary parathyroid glands. The Perrier classification scheme aids in communication of the parathyroid location to the surgeon.