Embryology and Anatomy of Parathyroid Gland

Normal parathyroid glands originate from the third and fourth pharyngeal pouches and are of endodermal origin.4 The inferior parathyroid glands and the thymus are derived from the third pharyngeal pouch, whereas the superior parathyroid glands arise from the fourth pharyngeal pouch.5 Because third pharyngeal pouch derivatives (i.e., the inferior parathyroid glands and thymus) migrate farther, these parathyroid glands are more likely to be in ectopic or aberrant positions than are the superior parathyroid glands. Despite this observation, about 80% of the inferior parathyroid glands are situated anterior to the recurrent laryngeal nerve on the lower (dorsal position) aspect of the thyroid gland, within 1 cm of where the inferior thyroid artery crosses the recurrent laryngeal nerve (Fig. 53-1). About 15% of lower parathyroid glands are found in the thymus or perithymic fat.6 Occasionally, parathyroid glands fail to descend. This results in an "undescended parathymus."7

The superior parathyroid glands are usually situated at the level of the cricoid cartilage. They are more dorsal in position than the inferior parathyroid glands. When not found at this site, they often descend caudally along the esophagus and sometimes (especially when abnormal) into the posterior mediastinum (Fig. 53-2). Superior parathyroid glands may be situated intrathyroidally, within the carotid sheath, or may also fail to descend and be situated in the pharyngeal musculature.8 Supernumerary parathyroid glands occur in about 20% of patients and can be situated in normal or ectopic sites. Supernumerary parathyroid glands situated at ectopic sites in patients with multiple adenomas are a common cause of persistent HPT. When four normal parathyroid glands are identified in a patient with primary HPT, the fifth supernumerary parathyroid gland is almost always situated in the mediastinum, usually within the thymus or perithymic fat.

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