Development of the Thyroid Gland
Origin
The thyroid gland develops from two sources:
- Median thyroid anlage (thyroid diverticulum) — forms the bulk of the gland (follicular cells)
- Lateral thyroid anlage (ultimobranchial body) — contributes parafollicular C cells
Median Thyroid Anlage — Development of Follicular Cells
Site of Origin
- Arises as a midline endodermal thickening on the floor of the pharynx at the junction of the anterior 2/3 and posterior 1/3 of the tongue — the future foramen caecum (at the apex of the sulcus terminalis)
- This corresponds to the level of the 1st and 2nd pharyngeal pouches
Timeline
- Appears at the end of the 3rd week (day 24) of intrauterine life as a hollow outgrowth
- The diverticulum elongates caudally, initially remaining hollow (thyroglossal duct), then becomes solid
Descent
- The thyroid anlage descends in the midline in front of the pharynx and trachea, passing:
- Anterior to the hyoid bone (during development the hyoid forms around the thyroglossal duct, which passes through its substance)
- Then anterior to the thyroid and cricoid cartilages
- Reaches its final position anterior to the trachea at the level of C5–T1 by the 7th week
Thyroglossal Duct
- The track of descent is called the thyroglossal duct
- Normally obliterates completely by the 8th–10th week
- Foramen caecum persists as a small pit at the base of the tongue — the only normal remnant
- The duct passes through the hyoid bone (anterior or through the body of the hyoid — the most common relationship is through/around its body)
Differentiation
- From the 10th week: follicles begin to form; colloid appears in follicles
- Follicular cells become capable of trapping iodine and synthesising thyroglobulin
- 12th week: gland becomes functional; T4 detectable in fetal serum
Lateral Thyroid Anlage — Development of C Cells
- The ultimobranchial body arises from the 4th (and possibly 5th) pharyngeal pouch
- Fuses with the median thyroid anlage as it descends
- Contributes parafollicular (C) cells to the thyroid gland — these are of neural crest origin
- C cells are concentrated at the junction of the upper third and lower two-thirds of the lateral lobes (where the ultimobranchial body fused)
Summary of Origins
| Component | Embryological Source | Gives Rise To |
|---|---|---|
| Median thyroid anlage | Floor of pharynx (foramen caecum) | Follicular cells (T3, T4) |
| Ultimobranchial body | 4th pharyngeal pouch + neural crest | Parafollicular C cells (calcitonin) |
Congenital Anomalies
Thyroglossal Cyst
- Most common congenital anomaly of the thyroid
- Results from persistence and secretion of any part of the thyroglossal duct
- Location: Along the path of descent — most commonly at or just below the hyoid bone (subhyoid = 60%); also suprahyoid, at foramen caecum, intralingual
- Features: Midline swelling that moves upward on swallowing AND on protrusion of the tongue — this tongue-protrusion movement distinguishes it from other midline neck swellings
- Treatment: Sistrunk’s operation — excision of the cyst with the central part of the hyoid bone and the track up to the foramen caecum (to prevent recurrence)
Thyroglossal Fistula
- Usually results from infection and rupture of a thyroglossal cyst, or from incomplete excision
- Opens in the midline anterior to the trachea
- Discharges mucoid material
- Treatment: Sistrunk’s operation
Ectopic Thyroid
- Thyroid tissue found anywhere along the path of descent:
- Lingual thyroid — most common ectopic site; at the base of the tongue (foramen caecum region); appears as a midline mass at base of tongue; moves on swallowing and tongue protrusion
- Sublingual thyroid — below the tongue but above the hyoid
- Retrosternal/substernal thyroid — below normal position; may develop into substernal goitre
- In lingual thyroid: may be the only functioning thyroid tissue — always perform thyroid scan before excision
- Ectopic thyroid can undergo all pathological changes (goitre, carcinoma)
Accessory Thyroid Tissue
- Small rests of thyroid tissue along the thyroglossal tract
- Usually clinically insignificant
Agenesis / Dysgenesis of the Thyroid
- Failure of development → congenital hypothyroidism (cretinism)
- Now detected by neonatal TSH screening (Guthrie test)
- Treatment: lifelong thyroxine replacement
Abnormal Relationship with the Hyoid Bone
- Normally the thyroglossal duct passes through the body of the hyoid bone
- This is why Sistrunk’s operation removes the central body of the hyoid — to prevent cyst recurrence from duct remnants passing through the bone