Embryology of the thyroid

Fig.1: Thyroid origins in primitive pharynx, showing the pharyngeal pouches and their contributions to adult structuresEmbryology is the study of how the body's various organs and tissues develop while an embryo, from fertilisation of the egg, to birth. Organs and body tissues, like muscles, nerves, etc. all develop at different rates.

The thyroid gland is the first of the body's endocrine glands to develop, beginning in the third week of gestation, even before the nervous system has begun development. On day 16-17, the embryonic thyroid is visible as an outgrowth in the floor of the primitive buccal cavity (the site of the future mouth and pharynx).

The site where this epithelial proliferation of the floor of the pharynx occurs is known as the foramen caecum. In adult life the remains of this structure can be seen as a small blind pit at the junction of the anterior two thirds and posterior third of the tongue.

Additional lateral contributions to the embryonic thyroid originate from a portion of the fourth pharyngeal pouch and ultimobranchial body (Fig. 1). These fuse with the median component and in adult life become the tubercles of Zuckerkandl, first described by Emil Zuckerkandl in 1902. These lateral contributions are also the source of the thyroid's C-cells, which produce calcitonin, and can be the origin of a medullary cancer later in life.

Fig.2: Side view of thyroid development, showing thyroid descent from foramen caecum via thyroglossal duct to final position in the neckThe primitive thyroid descends in front of the pharyngeal gut along the thyroglossal tract. During this migration the thyroid is still connected to the tongue via the thyroglossal duct. This tubular duct later solidifies and subsequently obliterates entirely.

Further descent of the thyroid gland carries it anterior (in front) to the hyoid bone and, subsequently, anterior to the cartilages of the larynx (voicebox). As the thyroid gland descends, it forms its adult shape, with an isthmus connecting the two lateral lobes (see Anatomy).

The thyroid gland tissues merge and migrate to their final location, in front of the trachea (windpipe) by day 45-50 (Fig. 2). By day 70 after conception, the thyroid gland is well developed, and begins concentrating iodide and producing thyroid hormone.

Thyroglobulin (TBG), the precursor protein upon which thyroid hormone is produced and stored, is present as early as  day 29 in thyroid follicle cells. As the thyroid follicle cells mature, TBG levels are detected in fetal serum by week 11 and increase right through gestation. 

After migration of the thyroid to its final position the thyroglossal duct usually disappears, but ectopic thyroid tissue can be found anywhere along the thyroglossal tract. This can produce a number of problems in adult life, which are detailed on following pages in this section.