The condition is congenital and there is a female predilection 1-3.
Many patients are asymptomatic and the diagnosis is made incidentally either as a result of imaging the tongue or attempting to image the thyroid and noting that it is absent.
In symptomatic patients the lingual mass may result in dysphagia, bleeding from mucosal ulceration, or even air-way obstruction (more common in infants) 1-2.
Direct examination may reveal a nodular red mass of variable size, ranging from a few millimetres to 3-4cm 2.
Thyroid function tests are either normal (majority of patients) or demonstrate variable states of hypothyroidism (up to a third of patients) 2.
A lingual thyroid results from failure of the normal caudal migration of the thyroid from foramen caecum down to its normal location anterior to the larynx and upper trachea. Thyroid tissue may be found anywhere along the course of the thyroglossal duct, however complete arrest with thyroid tissue located at the base of tongue is most common, and represents 90% of all cases of ectopic thyroid 1-2. Microscopic deposits of thyroid tissue along its route of descent have been identified in up to 10% of the population, representing small amounts of tissue being 'left behind' during normal development 2.
The thyroid tissue is normal histologically and functionally.
Carcinoma of a lingual thyroid has been reported but is very rare, presumably no higher than normal thyroid.
Ultrasound is only of use in demonstrating absent thyroid tissue in the normal location, which is the case in the majority of cases 1. Only occasionally do patients have thyroid tissue both at the tongue base and elsewhere in the neck.
CT demonstrates are hyperdense soft tissue mass, of the same attenuation as normal thyroid tissue. It is hyperdense on account of the accumulation of iodine within the gland 1,3.
Following contrast administration, the entire gland demonstrates prominent homgenous enhancement (again just like the normal thyroid gland). There are occasional case reports of inhomogenous contrast enhancement 5.
Usually seen as a well defined mass with no invasive features
Signal characteristics include
- T1 - iso to hyperintense to muscle 1,3,6
- T2 - can vary from hypo to iso to hyperintense to muscle 3,6
- T1 C+ (Gd) - homogeneous contrast enhancement
A thyroid scan is excellent at not only confirming the diagnosis, but also identifying the presence of any thyroid tissue elsewhere in the neck.
Treatment and prognosis
Often no treatment is required. In cases where surgical excision is being contemplated, it is essential to establish if there is any normal thyroid tissue elsewhere (usually not the case) as removal of the lingual thyroid will in most cases render the patient profoundly hypothyroid 2.
A general differential for a posterior midline neck mass includes 3:
- 1. Larheim TA, Westesson P. Maxillofacial Imaging. Springer Verlag. (2008) ISBN:3540786856. Read it at Google Books - Find it at Amazon
- 2. Barnes L. Surgical pathology of the head and neck. Marcel Dekker. (2001) ISBN:0824701097. Read it at Google Books - Find it at Amazon
- 3. Lufkin RB, Borges A, Villablanca P. Teaching atlas of head and neck imaging. Thieme Medical Publishers. (2000) ISBN:0865776911. Read it at Google Books - Find it at Amazon
- 4. Willinsky RA, Kassel EE, Cooper PW et-al. Computed tomography of lingual thyroid. J Comput Assist Tomogr. 1987;11 (1): 182-3. Pubmed citation
- 5. Shah HR, Boyd CM, Williamson M et-al. Lingual thyroid: unusual appearance on computed tomography. Comput Med Imaging Graph. 1988;12 (4): 263-6. Pubmed citation
- 6. Takashima S, Ueda M, Shibata A et-al. MR imaging of the lingual thyroid. Comparison to other submucosal lesions. Acta Radiol. 2001;42 (4): 376-82. Pubmed citation