What is the basic normal anatomy and sonographic imaging appearance of the parathyroid glands?
The parathyroid glands are typically four in number and occur as a superior pair and an inferior pair. Supernumerary glands are seen approximately in 3% to 13% of patients, and fewer than four glands can occur. The superior glands are derived from the fourth branchial pouch along with the lateral lobes of the thyroid. The superior glands tend to be more consistent in location, with most (>90%) located on the posterior surface of the thyroid, toward the upper pole ( Figure 61-3 ). Infrequently, they may be located above the lateral lobe of the thyroid or rarely in the retropharyngeal (1%) or retroesophageal (1%) space or in the thyroid gland itself (0.2%). The inferior glands arise from the third branchial pouch along with the thymus gland and their location is more variable. Sixty percent can be found posterior to the thyroid gland at the lower pole, and 25% are located inferior to the thyroid gland along a migratory path that extends into the superior mediastinum including within the thymus. The inferior glands may fail to descend and may be located above the superior glands. Ectopic parathyroid glands are not unusual and may be found within the carotid sheath, within the aorticopulmonary window, posterior to the carina, within the pericardium, or within the posterior triangle of the neck. Most parathyroid glands individually measure 3 mm in anteroposterior diameter, 1 mm in lateral width, and 5 mm in craniocaudal length, and due to their small size are infrequently identified on neck US.