Anatomy

Although, classically, there are four parathyroid glands, two superior and two inferior (Fig. 16.5), this is variable and up to 10 glands have been identified in one patient. In addition, parathyroid rests of cells may be found along the line of descent of the parathyroids, especially into the thymus gland. These probably account for the lack of need for vitamin D or calcium replacement in some patients who undergo total parathyroidectomy.

Middle constrictor muscle

Hyoid bone

Inferior constrictor muscle

Thyroid gland

Middle constrictor muscle

Hyoid bone

Inferior constrictor muscle

Thyroid gland

Branch of superior thyroid artery

Superior parathyroid gland

Inferior parathyroid gland

Figure 16.5. Posterior surface of the thyroid gland showing the parathyroid glands.

Branch of superior thyroid artery

Superior parathyroid gland

Inferior parathyroid gland

Figure 16.5. Posterior surface of the thyroid gland showing the parathyroid glands.

The superior parathyroid glands tend to be constant in their position, in the vicinity of the site where the inferior thyroid artery crosses the recurrent laryngeal nerve.

The inferior parathyroid glands are more variable in position but in almost 90% of people, they are situated in the region of the lower pole of thyroid, upper cornua of thymus, or in the tract between.

However, the inferior glands may be situated some considerable distance from this area. The glands may not descend fully with the thymus, or may over-descend. They may be found therefore in the normal line of descent from above the upper pole of thyroid, anywhere to the lower pole of thyroid, down into the upper cornua of thymus and may even descend into the superior mediastinum. If arrested on descent behind the thyroid, they may take up a position within a cleft in the thyroid gland and become intra-thyroid.

A gland may deviate from its normal line of descent ending up in an ectopic position. Glands have been described in the neck between trachea and oesophagus, retro-oesophagus, laterally in the neck and within the carotid sheath. A gland may descend into the thorax and may be intra-thymic, or in the superior or anterior mediastinum and be extra-thymic. Individual reports have described glands found in the middle mediastinum, intrapericardium, subpleural, in the posterior mediastinum and even intra-pulmonary.

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