LabMed

Ectopic Thyroid Tissue

At a Glance

Ectopic thyroid is defined as any thyroid tissue not located in its usual position (i.e., anterior and lateral to the second, third, and fourth tracheal rings). During development, the thyroid gland descends from the foramen cecum at the base of the tongue to its location at the front of the trachea. This connection between the foramen cecum and the final location of the thyroid is via the thyroglossal duct, which closes by involution during fetal development.

Failure of the thyroid to migrate along the thyroglossal duct leads to a lingual thyroid situated at the base of the tongue. Incomplete migration can lead to a high cervical thyroid, and excessive migration can lead to a superior mediastinal or even paracardiac location for the ectopic thyroid tissue. Thyroid tissue in the midline region, not in the lymph nodes, is usually ectopic in nature.

There is some controversy whether ectopic thyroid tissue can exist as the so-called lateral aberrant thyroid tissue, since thyroid tissue in lymph nodes or in a lateral location is usually metastatic in nature and may originate from an occult primary. Thus, ectopic thyroid tissue may result from incomplete descent of the thyroid gland or from aberrant development of the thyroglossal duct lining epithelium. An example of the latter is the finding of ectopic thyroid tissue in the wall of a thyroid gland cyst. Hypothyroidism is common in patients with ectopic thyroid tissue, and it is related to thyroid dysgenesis during development.

A lingual thyroid is the most common location for ectopic thyroid tissue and represents about 90% of cases of ectopic thyroid tissue. Most cases go undetected, with only about 1 in 10,000 having clinical complaints. Although most cases are asymptomatic, lingual thyroids can be associated with symptoms of hypothyroidism in about 70% of cases or symptoms due to a mass effect (e.g., difficulty breathing, swallowing), which brings them to clinical attention.

What tests should I request to confirm my clinical Dx? In addition, what follow-up tests might be useful?

Testing is directed toward identification of thyroid tissue when surgical excision is being contemplated for symptomatic relief or exclusion of hypothyroidism if medical treatment is planned. Testing for hypothyroidism is done using standard lab tests that show a high thyroid-stimulating hormone (TSH) and low T4 or FT4. Thyroid scintigraphy is done using either I-123 or Tc-99m (technetium pertechnetate). Midline ectopic activity usually represents a lingual thyroid. Thyroglossal duct cysts are usually not identified by thyroid scintigraphy, because they do not typically contain sufficient functioning thyroid tissue.(Table 1)

Table 1.

Test Results Indicative of the Disorder
TSH FT4
high low

What Lab Results Are Absolutely Confirmatory?

I-123 uptake or Tc-99m scintigraphy is diagnostic.

Additional Issues of Clinical Importance

Before surgical excision of any ectopic thyroid tissue, or even a thyroglossal duct cyst, it is important to exclude that the lingual thyroid or thyroglossal duct cyst does not represent the sole remaining thyroid tissue in the body. If the lingual thyroid represents the sole thyroid tissue in the body, surgical excision could result in the need for lifelong thyroxine replacement therapy.

Although midline ectopic thyroid tissue is usually benign, rare malignancy (e.g., papillary adenocarcinoma) can develop within any ectopic thyroid tissue and its presence does not automatically suggest a metastatic origin.

Errors in Test Selection

Struma ovarii is the presence of thyroid tissue in an ovarian tumor. However, these patients have thyrotoxicosis, as opposed to hypothyroidism.

Errors in interpretation of test results

Use of a whole body I-123 scan, to detect focii of extra thyroidal tissue is based on the specificity of the thyroid tissue's ability to concentrate, organify (incorporate into T4), and retain I-123. This specificity can be degraded by the uptake of I-123 in other tissues, such as choroid plexus, salivary glands, and nasopharynx. Expert radiologic opinion is needed to exclude and to identify nonthyroidal uptake and biodistribution in physiologic sites and secretions.

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