Thyroid origin and the thyroglossal duct cyst
The thyroid begins at the foramen caecum, a small pit on the tongue, and a tube it leaves behind can fail to disappear and form a midline neck cyst.
Remember
A painless midline neck lump that rises when the tongue is stuck out is a thyroglossal duct cyst, a leftover of the tube that joins the thyroid to the tongue while the thyroid travels down the neck.
Memory aid
the cyst is a leftover of the thyroid's journey from tongue to neck, so it sits in the midline and follows the tongue
Use it to place the cyst: it lies along the route the thyroid travelled, in the midline, and it is tied to the tongue.
The thyroid gland begins as a thickening of endoderm (the inner embryonic layer) at the base of the tongue, at a small pit called the foramen caecum. The thyroid primordium (the first group of cells that will become the gland) then descends, meaning it travels down, through the midline of the neck to its usual position in front of the trachea.
While it travels, the thyroid stays joined to the tongue by a thin tube, the thyroglossal duct, which normally disappears by about week 10. If part of the duct persists (does not disappear), fluid can collect in the remnant (what is left of the duct) and form a thyroglossal duct cyst along the route.
The cyst presents as a painless lump in the midline of the neck. It rises when the tongue is stuck out and also moves up on swallowing, because the duct is attached to the tongue.
The cyst can become infected and form a fistula (an abnormal channel) that discharges onto the skin, and the usual treatment is complete surgical excision (removal).
Contrast it with a branchial cyst. That one comes from persistent remnants of the pharyngeal clefts (the outer grooves of the embryonic neck) and sits at the side of the neck, not in the midline.
Key facts
- Starting point
- the thyroid begins at the foramen caecum, a small pit on the tongue, near its base.
- Journey
- the thyroid descends in the midline of the neck to lie in front of the trachea, leaving the thyroglossal duct behind.
- Duct
- the thyroglossal duct normally disappears by about week 10.
- Sign
- the cyst rises when the tongue is stuck out and also moves up on swallowing, because the duct is attached to the tongue.
- Complication
- the cyst can become infected and form a fistula (an abnormal channel) that discharges onto the skin.
- Treatment
- the usual treatment is complete surgical excision.
- Contrast
- a branchial cyst comes from persistent remnants of the pharyngeal clefts and lies at the side of the neck.
Exam traps
- A lingual thyroid is thyroid tissue that never descended and sits at the back of the tongue, so it does not appear as a front neck swelling that moves on swallowing.
- A second branchial (pharyngeal) cleft remnant forms a cyst at the side of the neck and has no attachment to the tongue, so it does not rise when the tongue is stuck out.
- The thyroid does not begin in the anterior mediastinum (the front compartment of the chest); it begins at the base of the tongue and descends in the neck.
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