The parathyroids are small glands that secrete parathyroid hormone, the major hormone that controls serum calcium homeostasis in humans. Usually four glands are present, two on each side, but three to six glands have been found. Each gland normally weighs 30 to 40 mg, but they may be heavier if more fat is present. Because of their small size, their delicate blood supply, and their usual anatomic position adjacent to the thyroid gland, these structures are at risk of being accidentally removed, traumatized, or devascularized during thyroidectomy (10).
The upper parathyroid glands arise embryologically from the fourth pharyngeal pouch (Figs. 7, 8). They descend only slightly during embryologic development, and their position in adult life remains quite constant. This gland is usually found adjacent to the posterior surface of the middle part of the thyroid lobe, often just anterior to the recurrent laryngeal nerve as it enters the larynx.
Figure 7. A and B. Shifts in location of the thyroid, parafollicular and parathyroid tissues. C. Approximates the adult location. Note that what has been called the lateral thyroid is now commonly referred to as the ultimobranchial body, which contains both C cells and follicular elements. (From Sedgwick CE, Cady B: Surgery of the Thyroid and Parathyroid Gland, 2nd ed., Philadelphia, WB Saunders, 1980; adapted from Norris EH: Parathyroid glands and lateral thyroid in man: Their morphogenesis, histogenesis, topographic anatomy and prenatal growth. Contrib Embryol Carnegie Inst Wash 26:247-294, 1937.)
The lower parathyroid glands arise from the third pharyngeal pouch, along with the thymus; hence, they often descend with the thymus. Because they travel so far in embryologic life, they have a wide range of distribution in adults, from just beneath the mandible to the anterior mediastinum (Fig. 8) (11). Usually, however, these glands are found on the lateral or posterior surface of the lower part of the thyroid gland or within several centimeters of the lower thyroid pole within the thymic tongue.
Parathyroid glands can be recognized by their tan appearance; their small vascular pedicle; the fact that they bleed freely when biopsy is performed, as opposed to fatty tissue; and their darkening color of hematoma formation when they are traumatized. With experience, one becomes much more adept at recognizing these very important structures and in differentiating them from either lymph nodes or fat. Frozen section examination during surgery can be helpful in their identification.
Figure 8. Descent of the lower parathyroid. Whereas the upper parathyroid occupies a relatively constant position in relation to the middle or upper third of the lateral thyroid lobe, the lower parathyroid normally migrates in embryonic life and may end up anywhere along the course of the dotted line. When this gland is in the chest, it is nearly always in the anterior mediastinum. (From Kaplan EL: Thyroid and parathyroid. In Schwartz SI (ed): Principles of Surgery, 5th ed., New York, McGraw-Hill, 1989, pp 1613-1685. Copyright by McGraw-Hill, Inc. Used by permission of McGraw-Hill Book Company.)
