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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 treated surgically in most cases.
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During the fifth week of gestation, the parathyroid glands form from the third and fourth branchial pouches. The glands derived from the third pouches descend caudally in the neck along with the thymus gland (formed from the third branchial arch), while the glands from the fourth pouches remain stationary. 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.
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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, are most commonly found near the inferior poles of the thyroid gland. These are sometimes embedded within the superior aspect of the thymus gland or within the thyrothymic ligament.
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Deviation from the standard migratory pattern during embryogenesis can lead to aberrantly located glands in approximately 15 to 20% of patients, although this has been reported to be higher in individuals with renal hyperparathyroidism. Aberrant 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.
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While the majority of people have four parathyroid glands, approximately 2 to 5% have five or more glands. A similar percentage has fewer than four glands.
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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, an anatomic characteristic which distinguishes them from surrounding fat. The arterial branches supplying the glands can be variably positioned relative to the recurrent laryngeal nerve.
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Normal parathyroid glands, often bean shaped, appear a distinct yellowish brown, often described as caramel in color. Manipulation of the glands and their blood supply during surgery will change the color to a darker mocha brown. Healthy glands weigh 30 to 40 mg on average.
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Parathyroid hormone (PTH) impacts calcium homeostasis by targeting the kidneys, skeletal system, and gastrointestinal tract. The parathyroid chief cells are responsible for the production and storage of PTH. Hormone release by parathyroid glands is directly controlled by feedback inhibition of the glands by serum calcium.
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PTH acts to raise calcium levels by several mechanisms. It influences the kidneys to increase calcium reabsorption, promotes resorption and calcium release ...