The parathyroid gland is a key regulator of calcium homeostasis. While hyperparathyroidism is now most often diagnosed in asymptomatic patients, untreated it can have devastating effects on multiple organ systems. Primary hyperparathyroidism, the most common cause of hypercalcemia in nonhospitalized patients, is curable with surgery in most cases.
During the fifth week of gestation, the parathyroid glands form from the third and fourth branchial pouches. The inferior glands derived from the third pouches descend caudally in the neck along with the thymus gland (formed from the third branchial arch), while the superior glands from the fourth pouches travel a relatively shorter distance resulting in less variable final location. Knowledge of this embryologic migration, which results in the glands arising from the fourth pouches to be superiorly located and those from the third pouches to be inferior, is critical when searching for ectopically located glands. Additionally, because of their embryologic origins, the superior glands will always be located dorsal to the recurrent laryngeal nerve, whereas the inferior parathyroid glands will always be relatively ventral.
The superior parathyroid glands are typically closely associated with the posterolateral aspect of the superior poles of the thyroid gland. The inferior glands, which are more variable in positioning due to their longer embryologic migratory path, are most commonly found near the inferior poles of the thyroid gland. Inferior glands may sometimes be embedded within the superior aspect of the thymus gland or within the thyrothymic ligament.
Deviation from the standard migratory pattern during embryogenesis can lead to ectopically located glands in approximately 15% to 20% of patients, although this has been reported to be higher in individuals with renal hyperparathyroidism. Ectopic glands can occur in any location along their migration course and have been identified from the carotid bifurcation to the level of the aortic arch.
While the majority of people have 4 parathyroid glands, approximately 2% to 5% have 5 or more glands. These extra glands are referred to as “supernumerary.” A similar percentage of patients have fewer than 4 glands.
The blood supply to parathyroid glands is fairly constant. Both the superior and inferior glands are fed from branches of the inferior thyroid artery. Occasionally, superior glands are supplied by an anastomotic branch from the inferior to the superior thyroid artery or are fed by both arteries. These arteries enter the glands at their hila, almost invariably from a direction relatively medial to the gland. The arterial branches supplying the glands can be variably positioned relative to the recurrent laryngeal nerve.
Normal parathyroid glands, often bean-shaped and embedded in a small amount of fat, appear a distinct yellowish brown, often described as caramel in color. Manipulation of the glands and their blood supply during surgery may change the color to a darker mocha brown. Healthy glands weigh 30 to 40 mg ...