There are usually four parathyroid glands, arranged as superior and inferior pairs on either side of the midline closely related to the thyroid gland (Figure 17.1). These tan-coloured oval structures each normally measure 4-6 mm in maximum dimension and weigh around 50 mg. The superior parathyroid gland (derived from the fourth pharyngeal pouch) is fairly constant in position and lies on the posteromedial aspect of the superior thyroid pole. A few superior parathyroid glands are located medial to the upper pole or in the retropharyngeal or retro-oesophageal space. The inferior parathyroid glands, derived from the third pharyngeal pouch are less constant in location, although they tend to be symmetrical bilaterally. Over half are found around the inferior pole of the thyroid lobe although other common locations include the thymus or high up on the anterior aspects of the thyroid lobe. Occasionally they may be located in the mediastinum or rarely in association with the roots of the great vessels.

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