7.0 Embryology and Anatomy of the Thyroid Gland
Key Features
The thyroid is an H-shaped vascular gland located in the anterior lower neck just below the larynx.
Its function is the synthesis and secretion of thyroid hormones thyroxine (T4) and triiodothyronine (T3).
The isthmus overlies the second to fourth tracheal rings.
The thyroid weighs, on average, 25 g in adults.
A pyramidal lobe may be present as an extension of the embryologic thyroglossal duct (50%).
The tubercle of Zuckerkandl is a lateral or posterior projection from the thyroid lobe.
The thyroid gland is usually not visible externally; when it increases in size, it forms a characteristic swelling in the neck called a goiter.
Embryology
The human thyroid gland begins to develop at around 4 weeks after conception and moves down the neck while forming its characteristic bilobular structure, which is completed by the third trimester ( Fig. 7.1 ).
Ligaments and Fascia
The thyroid gland is ensheathed by the middle layer of the deep cervical fascia. The posteromedial part of the gland is attached to the cricoid cartilage, first and second tracheal rings, by the posterior suspensory ligament (Berry ligament). Under the layer of cervical fascia the gland has an inner true capsule, which is thin and adheres closely to the gland.
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