A normal parathyroid gland is approximately 3 mm x 3 mm x 3 mm. This is the size of a grain of rice. Its color is yellow-brown and it weights approximately 40 – 60 mg. Normally, the parathyroid glands are located in the posterolateral capsule of the thyroid. However, ectopic location of one or more parathyroid glands may occasionally be encountered. Embryology may help in understanding the potential locations of an ectopic parathyroid adenoma.
Embryology of the parathyroid glands
Embryologically, the parathyroid glands are derived from the 3rd and the 4th pouches between the branchial arches arising during the 4th to 5th weeks of development. The upper glands arise from the lower or 4th pouch. The lower glands develop from the higher or 3rd pouch and descent lower into the neck as the embryo matures. Because the upper glands travel a shorter distance during embryologic development, they are more consistent in their location.
Common ectopic locations
Superior parathyroid glands
Potential ectopic locations of the superior parathyroid glands include:
- above the level of the thyroid cartilage
- in the paraesophageal or retroesophageal / retrolaryngeal / retropharyngeal space
- deep in the posterior mediastinum
- within the thyroid parenchyma
Inferior parathyroid glands
Due to its different embryologic origin, there is more variation in the ectopic location of an inferior parathyroid gland. Lower parathyroids can be found anywhere from the angle of the mandible to the pericardium. Common, however, locations include:
- cranial to the superior pole of the thyroid
- inside the carotid sheath
- in the upper mediastinum / intrathoracic
- within the thymus
- in the thyro-thymic ligament
- within the thyroid parenchyma (intrathyroid)
Ectopic location of one or more parathyroid adenoma may result in failure of surgery for primary hyperparathyroidism. This is highly probable if preoperative localization studies failed to identify their ectopic location. Therefore, unrecognized ectopic parathyroid adenoma may be the cause of persistent/recurrent primary hyperparathyroidism.
Careful preoperative investigation, using sophisticated localization studies, is required to accurately identify the precise location of ectopic adenoma. Despite that ultrasonography and Tc99m-sestamibi scan remain the two basic localization studies, in some complicated cases more extensive investigation, using modern imaging methods, may be required. These imaging methods include computed tomography (CT), 4D-CT, Magnetic Resonance Imaging (MRI) and SPECT. Successful and accurate preoperative localization is required before reoperative surgery for persistent / recurrent hyperparathyroidism.
The surgeon should be highly experienced in endocrine surgery to achieve cure of these patients.