An ectopic thyroid gland is one which is located in a location other than the normal position anterior to the laryngeal cartilages.
Ectopic tissue refers to the gland located outside its normal anatomical position whereas accessory tissue refers to extra tissue located remote from a normally positioned gland .
During embryological development, the thyroid gland migrates down from the foramen cecum at the posterior aspect of the tongue to its permanent location in the infrahyoid neck. This normal migration can be halted at any point, or indeed can go 'off-target' with thyroid tissue coming to rest in unusual locations within the neck or elsewhere :
- lingual: base of tongue (90%)
- sublingual: below the tongue
- other sites, e.g. mediastinum (<1%), intratracheal
Thyroid tissue may also be present in the normal thyroid bed, so-called "dual ectopic thyroid" .
As imaging findings (other than location) are identical irrespective of location, the discussion on ectopic thyroids is continued as part of the lingual thyroid article.