Development of Thyroid
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Direct answer
The thyroid is the first endocrine gland to appear: in the fourth week a median endodermal diverticulum grows down from the floor of the pharynx at the foramen caecum — between the first and second arch swellings — and descends in front of the hyoid and laryngeal cartilages, trailing the thyroglossal duct, to reach its final position by week seven. The duct normally involutes, leaving the foramen caecum at its apex and, in up to half of people, a pyramidal lobe at its lower end. Follicular cells therefore come from the tongue endoderm; the calcitonin-secreting parafollicular C cells arrive separately, from neural crest cells that colonised the ultimobranchary body of the caudal pharyngeal complex. The gland begins trapping iodine and making colloid at 10-12 weeks, which is why radioiodine and antithyroid drugs are hazards of early pregnancy, and why the midline descent path explains every thyroglossal cyst.
What you must remember
- Origin and timing: median endodermal thickening at the foramen caecum during week four; the diverticulum elongates into the thyroglossal duct and bilobates as it descends.
- Descent relations: the tract passes downward in front of (classically through or behind, in a minority) the hyoid bone, then along the thyroid cartilage — a hyoid-attached tract is the reason thyroglossal cysts move on tongue protrusion.
- Timing milestones: final position by week seven; colloid and follicles from week ten to eleven; iodine trapping and hormone synthesis from about 10-12 weeks; until then the foetus depends on transplacental maternal T4.
- Pyramidal lobe: persistent lower duct in up to half of individuals, ascending from the isthmus or a lateral lobe — an anatomical reason subtotal thyroidectomy leaves this slip behind.
- C cells: neural crest cells enter via the ultimobranchary body (fourth-fifth pharyngeal pouch region) and disperse as parafollicular cells; medullary thyroid carcinoma is their tumour, and MEN 2 screening is RET-based.
- Thyroglossal cyst: midline swelling anywhere from foramen caecum to sternal notch — subhyoid commonest; moves on swallowing and tongue protrusion; treatment is Sistrunk's operation — cyst, entire tract and the central hyoid — because remnants otherwise cause recurrence.
- Ectopic thyroid: failure of descent leaves a lingual thyroid at the base of the tongue — a midline mass that may be the patient's only thyroid tissue; scan before excision.
- Clinical anatomy of the midline: thyroglossal cysts are midline and move with protrusion; branchial cysts are lateral at the anterior border of sternocleidomastoid — the one-line differential every surgical viva wants.
Working through a midline neck lump in a teenager
A 15-year-old has a firm, rounded, painless swelling just below the hyoid that moved upward when she swallowed and — on asking her to protrude her tongue — jumped again. Both signs follow from the tract: swallowing moves laryngeal skeletons, and protrusion pulls the tract's anchorage at the tongue base and hyoid. A thyroglossal cyst sits on this tract anywhere from the foramen caecum down, is midline, and classically becomes symptomatic after an upper respiratory infection. Ultrasound must also confirm that a normal thyroid exists below it — a "cyst" is occasionally an ectopic thyroid, the patient's only functioning tissue; removing it unthinkingly creates lifelong hypothyroidism.
Definitive surgery is Sistrunk's: the cyst, the whole tract, and the central centimetre of hyoid body come out together — excising the cyst alone invites recurrence from the residual tract. Contrast the branchial cyst — lateral, at the anterior border of sternocleidomastoid, not moving on protrusion.
Where students slip
Students say the thyroid descends "behind the hyoid" only — the tract classically passes in front of, through, or behind the hyoid, and Sistrunk's operation exists precisely because of the through-route. They misattribute C cells to the third pouch alone — say "ultimobranchary body, from the caudal (fourth-fifth) pharyngeal complex, seeded by neural crest". And they forget the 10-12 week functional onset — the fact that makes radioiodine absolutely contraindicated in pregnancy. The prof favourite is "why does the cyst move on protrusion" — answer with the hyoid attachment, then name Sistrunk.
Frequently asked questions
From where does the thyroid diverticulum arise?
The foramen caecum of the tongue, between the first and second pharyngeal arch swellings, during the fourth week. It is visible even in the adult tongue at the apex of the sulcus terminalis.
Why does a thyroglossal cyst move on tongue protrusion?
Its tract remains attached to the foramen caecum and hyoid bone, so protruding the tongue tugs the cyst upward. Movement on swallowing is shared by all hyoid- and larynx-linked swellings.
What is the embryological origin of parafollicular C cells?
Neural crest cells that colonised the ultimobranchary body and dispersed within the thyroid as parafollicular cells. They secrete calcitonin and give rise to medullary carcinoma.
When does the foetal thyroid become functionally active?
At about 10-12 weeks, when iodine trapping and colloid formation begin; earlier foetal needs are met by transplacental maternal thyroxine. Radioiodine is therefore contraindicated throughout pregnancy.
What does the Sistrunk operation involve?
Excision of the cyst together with the entire thyroglossal tract and the central body of the hyoid. Leaving tract or hyoid remnant is the standard cause of recurrence.