THYROID GLAND

Embryology

The thyroid gland is formed from an ectodermal down-growth from the first and second pharyngeal pouches and descends to its final level just below the cricoid cartilage. The line of descent is known as the thyroglossal tract and, by the time of birth, this has normally involuted and disappeared. During descent, the thyroid gland comes in contact with the ultimobranchial bodies from the fourth pharyngeal pouches and cells from these are thought to form the parafollicular, calcitonin producing or ‘ C-cells’ of the thyroid. On reaching its final position, the thyroid divides into two lobes with a central bridge or isthmus. The site of origin forms the foramen caecum of the tongue.

Embryological abnormalities are:

• failure of descent – this may result in a lingual thyroid, an upper midline neck swelling, or a pyramidal lobe.

• cell nests in a persisting thyroglossal tract – these may result in a thyroglossal cyst.

• over descent resulting in a retrosternal goitre.

• agensis of one lobe – this may cause confusion on an isotope scan with a solitary nodule in one lobe and a cold area on the contralateral side.

Anatomy

The normal thyroid gland consists of two lateral lobes joined in the midline by a bridge of thyroid tissue or isthmus. The lobes extend from the level of the thyroid cartilage superiorly to the 5th or 6th tracheal rings inferiorly. The isthmus is variable in width and rests on the 2nd to 4th tracheal rings.