Anatomy

Most humans have four parathyroid glands. The percentage of individuals with supernumerary glands varies from 2.5% to 22%. The presence of as many as eight parathyroid glands has been reported, and different series have determined that there is a wide variation in the number of individuals with fewer than four glands. The exact number of individuals with fewer than four glands may be impossible to determine because the surgeon or researcher may not be able to find one or more glands, and a missing gland could represent an unobserved rather than an absent gland.

The parathyroid glands usually lie on the posterior surface of the thyroid gland, each with its own connective tissue capsule surrounded by lighter colored fat globules. Figures 38-2 and 38-3 depict the normal location of parathyroid glands with emphasis on their anatomic relations. The superior parathyroid gland is normally located on the posteromedial aspect of the thyroid gland near the tracheoesophageal groove. The majority of these glands are located within a circumscribed area 2 cm in diameter, about 1 cm above the intersection of the recurrent laryngeal nerve and the inferior thyroid artery. They may be either intimately associated with the cricothyroid junction or tucked behind the upper and middle thirds of the thyroid. When a gland is in intimate association with the cricothyroid junction, it is suspended by a small pedicle and enveloped by a pad of fatty tissue. When they are located on the posterior surface of the upper pole, parathyroid glands are invariably beneath a thyroid-investing fascial sheath. Superior parathyroid glands can be located farther down, sometimes obscured by the inferior thyroid artery or the recurrent laryngeal nerve. A rather unusual location is above the upper thyroid pole in the posterior aspect of the neck, the retropharyngeal or retroesophageal space. True superior intrathyroidal glands are rarely seen.

FIGURE 38-2. Frontal view of the anatomic location of parathyroid glands.

The inferior parathyroid glands are more widely distributed. They are normally located on the posterolateral aspect of the inferior pole of the thyroid gland, below the inferior thyroid artery, although they may be located anterior, inferior, or lateral to the inferior thyroid pole. They are usually surrounded by fat and sometimes may be in a fatty

FIGURE 38-3. Lateral view of the anatomic location of upper and lower parathyroid glands. A, Right. B, Left.

FIGURE 38-3. Lateral view of the anatomic location of upper and lower parathyroid glands. A, Right. B, Left.

appendage of the inferior thyroid pole. Some of these inferior glands can be found high up on the thyroid lobe. Another common location of the inferior parathyroids is the region inferior to the thyroid, close to the thyrothymic ligament or within the cervical part of the thymus. Inferior glands can also be located farther down in the thymus or in the fatty tissue of the anterior mediastinum, at the carotid bifurcation, or within the substance of the thyroid gland. Most anatomic studies have not involved serial sections of the thyroid gland, but Thompson and colleagues6 carefully sliced all thyroid lobectomy specimens during a 10-year period and found truly intrathyroid parathyroid glands in 3% of the cases. They were all located in the lower third of the thyroid and, therefore, were considered inferior parathyroid glands. Failure of an inferior parathyroid gland to descend during its embryonic development may result in a gland located higher up in the neck, even above the upper thyroid pole. These glands are usually surrounded by a remnant of thymic tissue. When supernumerary glands exist, the fifth gland is most often located in the thymus or in relation to the thyrothymic ligament.6"9 Figure 38-4 graphically demonstrates the frequency of anatomic locations of both superior and inferior parathyroid glands as reported by Gilmour in a study on 527 autopsies.10

As previously stated, parathyroid glands can be either extracapsular or intracapsular. When the gland is located underneath the fibrous capsule of the thyroid, it is designated intracapsular, whereas when it lies outside the capsule it is termed extracapsular. This anatomic feature has great surgical importance. When an intracapsular gland is diseased, it expands locally within the confines of the thyroid capsule and remains in its place. An enlarged extracapsular parathyroid gland, on the other hand, tends to be displaced to the area of least resistance. Thus, an extracapsular gland at the cricothyroid junction falls into the posterior mediastinum and an extracapsular gland within the thymus disappears behind the clavicle and falls into the superior anterior mediastinum.

Symmetry of parathyroid glands varies for parathyroids III and IV. Symmetry of superior glands is found in approximately 80% of the cases, whereas approximately 70% of inferior glands are symmetrical. Relative symmetry of all four glands is noted in approximately 60% of the cases. It is important to note that symmetry is less marked when the glands are located in an unusual site.7

When two parathyroid glands are intimately related to each other and appear to be fused, they are known as "kissing pairs." This is a rare finding. A kissing-paired parathyroid can be differentiated from a bilobular gland by the presence of a cleavage plane present in the kissing pair and an intact capsule in the bilobulated gland.

The parathyroid glands vary in size, shape, and color. They are spherical, somewhat flattened, or ovoid bodies whose shapes are modeled by pressure from the surrounding structures. The size of parathyroid glands varies from 4 to 6 mm in length and 3 to 4 mm in width. The average parathyroid gland is about 5x3x1 mm. When they are long, they tend to be narrow and thin. Conversely, when they are short, they are wide and thick. The average weight of a parathyroid is 35 to 40 mg, but it ranges from 10 to 70 mg. The color of the glands varies with age. In the newborn, they are gray and semitransparent. They are light pink in children,

UPPER PARATHYROID

LOWER PARATHYROID

UPPER PARATHYROID

LOWER PARATHYROID

FIGURE 38-4. Frontal view of the anatomic location of upper and lower parathyroid glands as reported by Gilmour.10

turning yellow in adults as their fat content increases. In older adults, they become darker.6"11

Parathyroid glands may conceivably be confused with small lobules of fat, with accessory nodules of thyroid tissue, or even with lymph nodes. Several physical characteristics may help to distinguish one from the other. The parathyroid glands are faintly globular or oblong structures that are softer in consistency than the adjacent thyroid or lymph nodes. Fat lobules are more friable than parathyroid glands and do not have the gland consistency or the lace-work of blood vessels on the surface. Lymph nodes have a more rounded configuration and are more adherent to the surrounding tissues. Thyroid nodules are always harder, more reddish, and less homogeneous than parathyroid glands. Parathyroid tissue is quite vascular, and on biopsy a "blush" or diffuse bleeding can be seen on the cut surface. Neither fat nor lymph nodes exhibit such a blush.61213 At surgery, the typical appearance of a parathyroid gland is that of a small "body" that moves inside its own fat capsule when gentle pressure on the surface is applied with a fine surgical instrument.

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