Which is the most common site of ectopic thyroid tissue in the body?

Explanation

## Ectopic Thyroid Tissue — Most Common Sites **Key Point:** Ectopic thyroid tissue arises from incomplete descent of the thyroid gland along the thyroglossal duct during embryonic development (weeks 3–7). ### Embryological Basis The thyroid develops at the base of the tongue (foramen caecum) and descends through the neck to reach its final position in the lower neck by week 7. Failure of complete descent or persistence of the thyroglossal duct results in ectopic thyroid tissue. ### Sites of Ectopic Thyroid — Frequency Ranking | Site | Frequency | Clinical Significance | | --- | --- | --- | | **Foramen caecum (lingual thyroid)** | 70% of ectopic thyroid | Most common; may be only thyroid tissue present | | Thyroglossal duct cyst | 15–20% | Contains thyroid tissue within cyst wall | | Sublingual/submandibular region | 5–10% | Along the path of descent | | Anterior mediastinum | 2–5% | Rare; lowest in descent pathway | **High-Yield:** Lingual thyroid (foramen caecum) accounts for approximately **70% of all ectopic thyroid tissue**. It is located at the junction of the anterior two-thirds and posterior one-third of the tongue. ### Clinical Pearls - **Lingual thyroid** may be the only functioning thyroid tissue in 70% of cases — thyroid scan essential before surgical removal. - Risk of hypothyroidism if ectopic tissue is removed without confirmation of normal thyroid gland presence. - Presents as a midline mass at the base of the tongue; may cause dysphagia, dysphonia, or dyspnea if large. - **Thyroglossal duct cyst** is the most common congenital neck mass overall, but contains thyroid tissue only in the cyst wall, not as the primary ectopic site. **Warning:** Do not confuse thyroglossal duct cyst (a cyst containing thyroid tissue) with lingual thyroid (ectopic thyroid tissue itself). Lingual thyroid is the most common site of ectopic thyroid tissue *per se*. ### Embryological Pathway ```mermaid flowchart TD A[Thyroid primordium at foramen caecum]:::outcome --> B[Week 3-4: Begin descent]:::action B --> C[Passes through epiglottis]:::action C --> D[Enters anterior mediastinum]:::action D --> E[Ascends to final position in lower neck]:::action E --> F[Normal thyroid gland]:::outcome B -.->|Incomplete descent| G[Lingual thyroid<br/>70% of ectopic cases]:::urgent C -.->|Persistence of duct| H[Thyroglossal duct cyst<br/>15-20%]:::outcome D -.->|Rare descent failure| I[Mediastinal ectopic thyroid<br/>2-5%]:::outcome ``` [cite:Gray's Anatomy 42e Ch 9]