Ectopic Thyroid

In some individuals, reminders of the embryological origins of the thyroid can persist into later life, presenting in various ways depending on the site of the problem in the neck or chest. Generally there is a close anatomical relationship to the site of the thyroglossal tract (its embryology is described here).

Fig.1: Diagram of ectopic sites of thyroid tissueThe anatomical and pathological findings related to embryological development which are encountered in adult life are summarised below (Fig. 1):

  • foramen caecum In adult life the remains of this structure can be seen as a small blind pit at the junction of the anterior two thirds and posterior third of the tongue.

  • lingual thyroid When the thyroid fails to descend into the neck it can remain at the back of the tongue as a lump.

  • thyroglossal cyst is detailed on its own webpage here

  • pyramidal lobe A pyramidal lobe of the thyroid may be observed in as many as 50% of patients; this structure represents a persistence of the inferior end of the thyroglossal duct that has failed to obliterate. It can be of variable length and is found attached to the top of the isthmus of the thyroid or to the adjacent thyroid pole.

  • ectopic thyroid rests Thyroid rests are deposits of ectopic thyroid tissue usually found in the thyrothymic area below the thyroid. 

Several of these findings, often found at the time of a thyroidectomy, are described in more detail below

Lingual Thyroid

This is a rare anomaly occurring in about 1 in every 100,000 people, but of all the ectopic thyroids 90% are lingual thyroids. Lingual thyroid occurs where the thyroid fails to descend from its origin at the back of the tongue (foramen caecum) into the neck during embryological life. It is important before contemplating removal, that two thirds of affected patients have no thyroid tissue in the neck.

Fig.2: Lingual thyroid, with a mass at the back of the tongueThe presentation is usually with a lump at the back of the tongue, which may impede swallowing or breathing if large, but most patients are unaware of its existence (Fig. 2). Ectopic thyroid is usually insufficient as a source of thyroid hormone, and up to 70% have hypothyroidism (low thyroid state).

Diagnosis is made by demonstration of the mass at clinical examination, but imaging is necessary to confirm that this is the only site of thyroid tissue, especially if removal is contemplated. Removal of the lump without knowing whether there is any other source of thyroid hormone can lead to hypothyroidism.

Fig.3: Thyroid scan demonstrating lingual thyroid as only site of thyroid tissueCT scan and MRI scanning can show the mass at the base of the tongue, extending down behind it and resembling a thyroglossal cyst. The best method of imaging however, is to use a radionuclide scan, such as a technetium scan (Fig. 3), which can demonstrate the site of the ectopic thyroid. 

If the lump is removed, histopathology of a lingual thyroid reveals a nonencapsulated collection of embryonic or mature thyroid follicles that can extend between the muscle bundles of the tongue. It is also possible to see parathyroid tissue with the lingual thyroid.

Thyroxine therapy corrects the hypothyroidism, but removal of the lump is usually not necessary unless it is large and symptomatic. Occasionally, large blood vessels on the surface can bleed or ulcerate necessitating treatment. 

Surgery or radioactive iodine ablation is warranted in this case, and the latter is very effective. Imaging with radionuclide scanning must be done before surgical removal however, to ensure that there is another source of thyroid hormone in the neck, and prevent the onset of hypothyroidism.

Pyramidal Lobe

Fig.4: Pyramidal lobe enlargement (arrowed) in large goitre specimenFig.5: Thyrothymic thyroid rest (arrowed) attached to the bottom of the right thyroid lobe and extending down into the chestThe pyramidal lobe of the thyroid may be observed in as many as 50% of patients (Fig. 4).

This structure represents a persistence of the inferior end of the thyroglossal duct that has failed to obliterate. It is usually a thin conical structure, can be of variable length and is found attached to the top of the isthmus of the thyroid or to the adjacent thyroid pole. It is more commonly attached to the left side of the isthmus than the right.

The pyramidal lobe may extend superiorly as far as the hyoid bone, and be attached to it by a fibrous strand. It is important to make sure the pyramidal lobe is removed during thyroidectomy, to prevent recurrence of goitre, Graves' disease or cancer in the remnant.

Thyroid Rests

Thyroid rests, or more strictly thyrothymic thyroid rests are deposits of thyroid tissue arising in the thyrothymic tract below the thyroid lobes. They can occur on one or both sides are are of variable size, although most are small. Occasionally they can be very large and turn up on a chest XRay or CT scan, or cause symptoms such as swallowing or breathing difficulty due to pressure on other structures (Fig. 5).

Thyroid rests tend to be attached by a thin piece of thyroid or fibrous tissue to the main body of the gland, but about 20% have no attachment at all and can migrate into the chest, presenting as an unexplained intrathoracic mass. More usually however, they are a curiosity removed with rest of the thyroid at the time of a total thyroidectomy, via the standard neck incision. If completely separate, then a manubriotomy might be needed to gain access to the mass in the chest for removal.