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Developmental anomalies
There are two parathyroids, superior and inferior, on each side. They
The thyroid develops as an outgrowth from the floor of the pharynx
(thyroglossal duct) in the position of the future foramen caecum of the
are about the size of a pea and are embedded in the back of the thyroid
(but outside its capsule). They develop from the third (inferior parathy-
tongue. It descends ventral to the larynx before dividing into two lobes. The parathyroid glands
The stem of the outgrowth, the thyroglossal duct, normally disappears roid) and fourth (superior parathyroid) pharyngeal pouches of the
although it may remain in part. Aberrant thyroid tissue may occur any- embryo. The thymus also develops from the third pouch and may drag
where along the course of the duct or thyroglossal cysts may appear. the inferior gland down with it so that the adult gland may lie in the
Such thyroid remnants move upwards when the tongue is protruded. thorax.
Infection of a persistent thyroglossal duct may occur, when the duct
must be excised.

The oesophagus and trachea and the thyroid gland 143