- Email: [email protected]
Anatomy and Embryology of the Thyroid and Parathyroid Glands
Anatomy and Embryology of the Thyroid and Parathyroid Glands Bruno A. Policeni, MD,* Wendy R.K. Smoker, MD, FACR,* and Deborah L. Reede, MD† The thyroid and parathyroid glands are cervical endocrine glands responsible for metabolism-related functions. Radiologists are frequently asked to evaluate pathology related to the thyroid and parathyroid glands before planned surgical intervention. Knowledge of embryology and anatomy is fundamental in this region because rather complex underlying embryology produces substantial anatomic variation both in the thyroid bed and elsewhere in the neck and mediastinum. Semin Ultrasound CT MRI 33:104-114 © 2012 Elsevier Inc. All rights reserved.
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he thyroid and parathyroid glands are endocrine glands that are normally located in the neck and are, as are all endocrine glands, characterized by lack of ducts and by discharge of their secretions directly into the bloodstream. They each have important functions in the regulation of body physiology. The thyroid gland is one of the largest endocrine glands and produces thyroid hormones, mainly triiodothyronine (T3) and thyroxine (T4). These hormones are directly related to body metabolism and play a particularly important role in brain maturation during fetal development.1 The thyroid gland also produces calcitonin, which is involved in calcium homeostasis. The parathyroid glands produce parathyroid hormone, which is involved in calcium homeostasis by targeting the kidneys, skeletal system, and intestines. Radiology plays an important role in the evaluation of these glands, with multiple imaging options available, as discussed in upcoming chapters in this edition. It is important to understand the embryology and anatomy of these glands to have a complete understanding of the congenital anomalies and multiple pathologies that may affect them.
*Department of Radiology, University of Iowa Hospital and Clinics, Iowa City, IA. †Department of Radiology, State University of New York, Downstate Medical Center, Brooklyn, NY. Address reprint requests to Bruno A. Policeni, MD, Department of Radiology, University of Iowa Hospital and Clinics, 200 Hawkins Drive, 3873 JPP, Iowa City, IA 52242. E-mail: [email protected]
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0887-2171/$-see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2011.12.005
Thyroid Gland Embryology The thyroid gland originates from a diverticulum located in the median ventral wall of the pharynx. Around the fourth week of development, an endodermal thickening appears in the midline floor of the primitive pharynx between the first and second pharyngeal pouches, dorsal to the aortic sac (Fig. 1). The primitive thyroid derives from this diverticulum, which is first spherical and then becomes lobulated. For the next 3 weeks, the primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends anterior to the hyoid bone and laryngeal cartilages to reach the lower neck. The pathway of the diverticulum retracts to form the thyroglossal duct (TGD), which later involutes. The TGD has its pharyngeal attachment to the ventral floor of the pharynx, where it contributes to formation of the tongue. As the thyroid tissue extends down the TGD, it passes just anterior to the hyoid bone and laryngeal cartilages (Fig. 2). The thyroid gland is initially spherical and then assumes a more bilobed configuration as it enlarges. It reaches its final destination in a pretracheal location by the end of the seventh week of gestation. Early in the fifth week of gestation, as mentioned earlier, the proximal portion of the TGD retracts and can be identified, in most adults, as a permanent pit at the apex of the sulcus terminalis on the dorsum of the tongue known as the foramen cecum (Fig. 2). Distally, the duct forms the pyramidal lobe of the thyroid gland. Part of the thyroid gland originates from the fourth and fifth pharyngeal pouches. The neural crest cells (ultimobranchial bodies) of these pouches contribute parafollicular C cells that produce calcitonin. The thyroid gland is able to function by the end of the third gestational month.
Anatomy and embryology of thyroid and parathyroid
105 puted tomography (CT), these lesions have similar signal characteristics to the normal gland demonstrating hyperdensity on CT, due to high iodine content, and high signal on T1- and T2-weighted MR sequences compared with adjacent tongue musculature. Lingual thyroid tissue is more conspicuous on T2-weighted sequences (Fig. 3). Ectopic thyroid tissue is subject to the same pathologic processes as the normal gland. Cancer within lingual thyroid tissue is extremely rare, appearing similar both clinically and radiographically to benign lingual thyroid tissue, thus requiring biopsy7 (Fig. 4).
Pyramidal Lobe Figure 1 Thyroid embryology. Thyroid primordium derives from an endodermal thickening that forms a diverticulum in the midline floor of the primitive pharynx between the first and second pharyngeal pouches. This will develop in the thyroid, which will migrate caudally along the thyroglossal duct (TGD). (Color version of figure is available online.)
During embryologic development, abnormal descent of the thyroid gland can occur and ectopic thyroid tissue can be found anywhere from the base of the tongue to the diaphragm. This is an important entity to recognize, as this ectopic tissue may be mistaken for neoplastic disease.2
As the TGD involutes, a small amount of residual tissue may persist distally to form the pyramidal lobe. The incidence of this lobe was found to be 55% in a postmortem study.8 It is important to identify this anatomic variant to avoid leaving residual tissue when total thyroidectomy is performed. The pyramidal lobe may branch from the right or left lobe, the isthmus, or from only one lobe with absence of the isthmus (Fig. 5A). On imaging studies, the pyramidal lobe demonstrates imaging characteristics similar to the normal thyroid gland. The shape of the lobe is variable, ranging from short and thick to long and thin (Fig. 5). The pyramidal lobe is most commonly identified in Graves’ disease.9
Lingual Thyroid The foramen cecum at the base of the tongue is the most common ectopic thyroid location, accounting for 90% of cases. Up to 10% of ectopic thyroid tissue is found in a variety of additional locations, including the sublingual space, TGD, mediastinum, heart, and esophagus.3 The origin of those locations outside the TGD pathway is probably related to the ultimobranchial bodies of the fourth pouch. This theory, however, is debated because it is not completely clear whether these cells have the potential to form follicular cells, with some authors believing that unusual locations of ectopic thyroid tissue may simply be related to aberrant migration.4 The incidence of lingual thyroid tissue in clinical studies is estimated to range between 1 in 3000 and 1 in 100,000 cases; however, postmortem studies have shown an incidence of approximately 10%.5 The underlying cause for this arrest is not completely understood. The lingual thyroid presents clinically as a firm, pinkish, painless mass covered by epithelium at the base of the tongue.6 Lingual thyroid tissue is associated with a topic thyroid gland in only 30% of cases. In the remaining 70% of cases, the lingual thyroid is the only thyroid tissue present without an associated topic gland. Ectopic thyroid tissue is usually imaged with nuclear scintigraphy, which not only demonstrates uptake in the lingual ectopic thyroid but also assesses whether there is any normal functioning tissue within the expected topic thyroid location. On magnetic resonance (MR) and com-
TGD Cyst Similar to ectopic thyroid tissue, a TGD cyst can occur anywhere from the foramen cecum to the lower midline of
Figure 2 Primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends along the TGD. The duct travels just anterior to the hyoid bone and laryngeal cartilages. Ectopic thyroid tissue and TGD cyst can be present anywhere along this pathway. (Color version of figure is available online.)
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Figure 3 Magnetic resonance imaging of a patient with difficulty in swallowing demonstrates a heterogeneous legion with areas of high signal on T2-weighted imaging (arrow, A), slight hyperintensity on T1-weighted imaging (arrow, B), and postcontrast enhancement (arrow, C) in the base of the tongue. Note the lack of a normal thyroid gland in the expected location in the axial T1-weighted imaging of the neck (D). Pertechnetate scintigraphy demonstrates uptake in the base of the tongue without uptake in the thyroid bed in the neck. (E) These findings are consistent with a lingual thyroid.
Anatomy and embryology of thyroid and parathyroid
Figure 4 Axial computed tomographic (CT) images of neck at the level of the tongue base demonstrate a hyperdense lesion (A) with coarse calcification (B). Surgically proven benign adenoma in the ectopic lingual thyroid. (Images courtesy of Dr. Yoshimi Anzai, University of Washington.)
the neck. This lesion results from failure of TGD obliteration. TGD cysts are usually located in the midline, most commonly infrahyoid (25%-65%), but they may be suprahyoid (15%-50%), at the hyoid level (20%-25%) (Fig. 6), or even intralingual (3%)4,9-11 (Fig. 7). These lesions have cystic characteristics on imaging studies. If a solid component is identified, the diagnosis of associated carcinoma should be considered.
Thyroid Gland Anatomy The thyroid gland resides in the midline of the lower neck, following its descent from the base of the tongue along the TGD, as described earlier. The gland is composed of right
107 and left lobes, typically interconnected by an isthmus in the midline, lying anterolateral to the larynx and trachea at approximately the level of the second and third tracheal rings.12 The normal thyroid gland weighs approximately 30 g. It is slightly heavier in women and becomes enlarged during pregnancy.13 The thyroid gland resides within the visceral space, anterior to the prevertebral space, surrounding the trachea and lying posterior to the infrahyoid strap muscles (sternohyoid and sternothyroid) (Figs. 8-10). The thyroid gland is attached to the larynx and trachea within the visceral space and, therefore, moves with the larynx during swallowing.9 When the thyroid gland becomes enlarged, it may extend inferiorly into the superior mediastinum, commonly described as a retrosternal thyroid. The superior and inferior thyroid arteries provide blood supply to the thyroid gland. These vessels have many anastomoses providing a rich vascular supply to this gland. The inferior thyroid artery is a branch of the thyrocervical trunk that arises from the subclavian artery. It courses anterior to the vertebral artery and longus colli muscles. The superior thyroid artery is the first branch of the external carotid artery, arising just below the hyoid bone (Fig. 11). These vessels can usually be identified on MR and CT angiography. Venous drainage is in the form of a plexus that drains into the internal jugular and brachiocephalic veins. The middle and inferior cervical ganglia of the sympathetic chain provide sympathetic innervation to the thyroid gland, whereas the vagus nerve provides parasympathetic regulation. An important anatomic structure related to the thyroid gland is the recurrent laryngeal nerve (RLN). Damage to the RLN is the most common and serious complication of thyroidectomy.14 The nerve itself is usually not identified on imaging studies. However, knowledge of its course and variants is crucial. The RLN is a branch of the vagus nerve, usually arising in the upper thorax with a different course on each side of the neck. On the left, the RLN extends inferiorly, loops under the aortopulmonary window, and ascends in the left tracheoesophageal groove. On the right, the RLN loops under the right subclavian artery and ascends in the right tracheoesophageal groove. Both nerves enter the cricothyroid articulation through the fibers of the inferior pharyngeal constrictor muscle.15 Clinical information regarding the side of vocal cord paralysis is necessary, given these anatomic differences, so that the scan range can be properly protocoled (Fig. 12). The RLN may have a nonrecurrent pathway in patients with an aberrant right subclavian artery; in which case, the nerve branches from the vagus nerve and travels directly to the larynx. This is less commonly seen on the left side in patients with right aortic arch.16
Parathyroid Gland Embryology The parathyroid glands develop from the third and fourth pharyngeal pouches (Fig. 13). Usually 2 parathyroid
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Figure 5 Illustration (A) demonstrates multiple connection sites for the pyramidal lobe to the thyroid gland. Hyperdense mass with areas of hypodensity originating from the thyroid isthmus demonstrated in these postcontrast axial CT images (B-D), consistent with a residual pyramidal lobe in a patient with Graves’ disease. (Color version of figure is available online.)
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Figure 7 Round hypointense lesion without enhancement on sagittal postcontrast T1-weighted imaging (A) and hyperintense on axial T2-weighted imaging (B) in the base of the tongue, consistent with a TGD cyst.
Figure 6 Nonenhancing cystic lesion located posteriorly to the hyoid bone asymmetric to the left in these postcontrast axial (A), coronal (B), and sagittal (C) CT images, consistent with the most common location for a TGD cyst.
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Figure 9 CT normal anatomy. 1, right lobe thyroid; 2, left lobe thyroid; 3, strap muscle (sternohyoid); 4, common carotid artery; 5, vertebral artery; 6, jugular vein; 7, sternocleidomastoid muscle; 8, anterior scalene muscle; 9, longus coli muscle; 10, trachea; 11, thyroid isthmus; 12, esophagus; 13, thyroid cartilage; 14, cricoid cartilage; 15, vertebral body.
Figure 8 Magnetic resonance normal anatomy. 1, right lobe thyroid; 2, left lobe thyroid; 3, strap muscle (sternohyoid); 4, common carotid artery; 5, vertebral artery; 6, jugular vein; 7, sternocleidomastoid muscle; 8, anterior scalene muscle; 9, longus coli muscle; 10, trachea; 11, thyroid isthmus; 12, esophagus; 13, thyroid cartilage; 14, cricoid cartilage; 15, aortic arch; 16, vertebral body.
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Figure 10 Axial diagram of the neck demonstrates the anatomic location of the thyroid gland within the visceral space in the midline. The gland is located anterior to the trachea and prevertebral space and posterior to the strap muscles. The superior parathyroid glands are located in the dorsal aspect of the thyroid gland superior lobe. The carotid arteries and jugular veins are located lateral to the visceral space. (Color version of figure is available online.)
Figure 11 The thyroid gland is supplied by the superior thyroid artery (a branch of the external carotid artery; arrow) and the inferior thyroid artery (a branch of the thyrocervical trunk that branches off the subclavian arteries; arrowhead). Note also the venous drainage of the thyroid gland into the internal jugular and innominate veins bilaterally. (Color version of figure is available online.)
Figure 12 Patient presenting clinically with a left vocal cord paralysis. Axial contrast-enhanced CT image at the thyroid cartilage level (A) demonstrates enlargement of the left pyriform sinus (dot) with a thickened aryepiglottic fold (arrow), consistent with a left vocal cord paralysis. Axial (B) and coronal (C) contrast-enhanced CT images at the aortopulmonary window level demonstrates an atherosclerotic aortic aneurysm (arrows) projecting anterior into the aortopulmonary window, likely compressing the left recurrent laryngeal nerve.
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glands are located 1 cm below the lower thyroid lobe. They can be located anywhere between the angle of the mandible and the upper mediastinum (Fig. 15). The incidence of intrathyroidal parathyroid tissue is quite low, approximately 2%.9 Arterial supply to parathyroid glands is from the superior and inferior thyroid arteries that supply the superior and inferior glands, respectively. The venous drainage is usually to thyroid veins. These glands are very small, measuring approximately 6 mm in craniocaudal dimension and 3-4 mm in transverse dimension, with a flattened “oval disk” appearance. When normal in size, these are not usually identified on most imaging studies. In contrast, parathyroid adenomas and gland hyperplasia are larger and more readily identified on imaging studies.
Conclusions Figure 13 The superior parathyroid gland originates from the fourth brachial pouch along with the ultimobranchial body that will contribute to the development of part of the thyroid gland. The inferior parathyroid glands originate from the third brachial pouch along with the thymus. (Color version of figure is available online.)
glands develop on each side, but this can range from 2 to 6 glands, with some authors reporting up to 12 glands.9,17 The cranial dorsal aspect of the third pharyngeal pouch generates the inferior parathyroid glands; the thymus also arises from the third pharyngeal pouch. The relationship of the inferior parathyroid glands and the thymus explains the variable position of the inferior parathyroid glands, as the thymus has a long course of descent into the superior mediastinum and frequently presents in ectopic locations. The superior parathyroid glands originate from the fourth pharyngeal pouch, along with part of the thyroid gland, as described previously.11 These glands start developing and migrating caudally during the fifth and sixth weeks of gestation.18,19
With the advent of minimally invasive surgical techniques, radiologists are more frequently asked to evaluate pathologies that affect the thyroid and parathyroid glands and to determine their specific anatomic location. Although the anatomy of these glands may appear simple, both are subject to considerable variability, primarily related to their somewhat complex embryology. Familiarity with the spectrum of anatomic locations and imaging appearances reflecting this embryologic variation will aid the radiologist in accurate interpretation.
Parathyroid Gland Anatomy The anatomic location of the superior parathyroid glands is relatively constant due to the close relationship between these glands and the thyroid gland. They typically reside on the dorsal aspect of the upper thyroid lobes at the level of the inferior border of the cricoid cartilage. The inferior parathyroid glands have a more variable location due to their embryologic relationship to the thymus, as discussed earlier. Fifty percent of the time, the inferior parathyroid glands are located along the lateral lower pole of the thyroid gland (Fig. 14). Fifteen percent of the time, these
Figure 14 Posterior schematic of the neck demonstrates the common locations for the parathyroid glands. Note the close proximity of the recurrent laryngeal nerves and the thyroid gland. (Color version of figure is available online.)
Anatomy and embryology of thyroid and parathyroid
Figure 15 Enhancing mass (arrow) in the superior mediastinum, just anterior to the aorta and posterior to the sternum on this postcontrast axial (A), sagittal (B), and coronal (C) CT images, consistent with an ectopic parathyroid adenoma. Tc-99m sestamibi study (D) demonstrates uptake in the superior mediastinum (arrows) at 15-minutes delay, which is persistent at 90 minutes, consistent with the ectopic parathyroid adenoma.
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References 1. Kester MH, Martinez de Mena R, Obregon MJ, et al: Iodothyronine levels in the human developing brain: Major regulatory roles of iodothyronine deiodinases in different areas. J Clin Endocrinol Metab 89: 3117-3128, 2004 2. Jones JA: Lingual thyroid. Br J Oral Maxillofac Surg 24:58-62, 1986 3. Sargin H, Köseog˘ulları O, Sanli A, et al: Three cases of lingual thyroid and review of the literature. Turk J Endocrinol Metab 3:131-135, 2003 4. Foley DS, Fallat ME: Thyroglossal duct and other congenital midline cervical anomalies. Semin Pediatr Surg 15:70-75, 2006 5. Hari CK, Kumar M, Abo-Khatwa MM, et al: Follicular variant of papillary carcinoma arising from lingual thyroid. Ear Nose Throat J 88:E7, 2009 6. Guneri A, Ceryan K, Igci E, et al: Lingual thyroid: The diagnostic value of magnetic resonance imaging. J Laryngol Otol 105:493-495, 1991 7. Kennedy TL, Riefkohl WL: Lingual thyroid carcinoma with nodal metastasis. Laryngoscope 117:1969-1973, 2007 8. Braun EM, Windisch G, Wolf G, et al: The pyramidal lobe: Clinical anatomy and its importance in thyroid surgery. Surg Radiol Anat 29: 21-27, 2007 9. Som PM, Curtin HD: Head and Neck Imaging (ed 4). St Louis, MO, Mosby, 2003 10. Allard RH: The thyroglossal cyst. Head Neck Surg 5:134-146, 1982
B.A. Policeni et al 11. Grevellec A, Tucker AS: The pharyngeal pouches and clefts: Development, evolution, structure and derivatives. Semin Cell Dev Biol 21:325332, 2010 12. Ozgur Z, Celik S, Govsa F, et al: Anatomical and surgical aspects of the lobes of the thyroid glands. Eur Arch Otorhinolaryngol 268:13571363, 2011 13. Gray H, Clemente CD. Anatomy of the Human Body (American ed 30). Philadelphia, PA, Lea and Febiger, 1985 14. Chiang FY, Lu IC, Chen HC, et al: Anatomical variations of recurrent laryngeal nerve during thyroid surgery: How to identify and handle the variations with intraoperative neuromonitoring. Kaohsiung J Med Sci 26:575-583, 2010 15. Ardito G, Revelli L, D’Alatri L, et al: Revisited anatomy of the recurrent laryngeal nerves. Am J Surg 187:249-253, 2004 16. Henry JF, Audiffret J, Denizot A, et al: The nonrecurrent inferior laryngeal nerve: Review of 33 cases, including two on the left side. Surgery 104:977-984, 1988 17. O’Rahilly R, Müller F. Human Embryology and Teratology (ed 2). New York, NY, Wiley-Liss, 1996 18. Moore KL, Persaud TVN. The Developing Human: Clinically Oriented Embryology (ed 6). Philadelphia, PA, Saunders, 1998 19. Safford SD, Skinner MA: Thyroid and parathyroid disease in children. Semin Pediatr Surg 15:85-91, 2006
T
he thyroid and parathyroid glands are endocrine glands that are normally located in the neck and are, as are all endocrine glands, characterized by lack of ducts and by discharge of their secretions directly into the bloodstream. They each have important functions in the regulation of body physiology. The thyroid gland is one of the largest endocrine glands and produces thyroid hormones, mainly triiodothyronine (T3) and thyroxine (T4). These hormones are directly related to body metabolism and play a particularly important role in brain maturation during fetal development.1 The thyroid gland also produces calcitonin, which is involved in calcium homeostasis. The parathyroid glands produce parathyroid hormone, which is involved in calcium homeostasis by targeting the kidneys, skeletal system, and intestines. Radiology plays an important role in the evaluation of these glands, with multiple imaging options available, as discussed in upcoming chapters in this edition. It is important to understand the embryology and anatomy of these glands to have a complete understanding of the congenital anomalies and multiple pathologies that may affect them.
*Department of Radiology, University of Iowa Hospital and Clinics, Iowa City, IA. †Department of Radiology, State University of New York, Downstate Medical Center, Brooklyn, NY. Address reprint requests to Bruno A. Policeni, MD, Department of Radiology, University of Iowa Hospital and Clinics, 200 Hawkins Drive, 3873 JPP, Iowa City, IA 52242. E-mail: [email protected]
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0887-2171/$-see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2011.12.005
Thyroid Gland Embryology The thyroid gland originates from a diverticulum located in the median ventral wall of the pharynx. Around the fourth week of development, an endodermal thickening appears in the midline floor of the primitive pharynx between the first and second pharyngeal pouches, dorsal to the aortic sac (Fig. 1). The primitive thyroid derives from this diverticulum, which is first spherical and then becomes lobulated. For the next 3 weeks, the primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends anterior to the hyoid bone and laryngeal cartilages to reach the lower neck. The pathway of the diverticulum retracts to form the thyroglossal duct (TGD), which later involutes. The TGD has its pharyngeal attachment to the ventral floor of the pharynx, where it contributes to formation of the tongue. As the thyroid tissue extends down the TGD, it passes just anterior to the hyoid bone and laryngeal cartilages (Fig. 2). The thyroid gland is initially spherical and then assumes a more bilobed configuration as it enlarges. It reaches its final destination in a pretracheal location by the end of the seventh week of gestation. Early in the fifth week of gestation, as mentioned earlier, the proximal portion of the TGD retracts and can be identified, in most adults, as a permanent pit at the apex of the sulcus terminalis on the dorsum of the tongue known as the foramen cecum (Fig. 2). Distally, the duct forms the pyramidal lobe of the thyroid gland. Part of the thyroid gland originates from the fourth and fifth pharyngeal pouches. The neural crest cells (ultimobranchial bodies) of these pouches contribute parafollicular C cells that produce calcitonin. The thyroid gland is able to function by the end of the third gestational month.
Anatomy and embryology of thyroid and parathyroid
105 puted tomography (CT), these lesions have similar signal characteristics to the normal gland demonstrating hyperdensity on CT, due to high iodine content, and high signal on T1- and T2-weighted MR sequences compared with adjacent tongue musculature. Lingual thyroid tissue is more conspicuous on T2-weighted sequences (Fig. 3). Ectopic thyroid tissue is subject to the same pathologic processes as the normal gland. Cancer within lingual thyroid tissue is extremely rare, appearing similar both clinically and radiographically to benign lingual thyroid tissue, thus requiring biopsy7 (Fig. 4).
Pyramidal Lobe Figure 1 Thyroid embryology. Thyroid primordium derives from an endodermal thickening that forms a diverticulum in the midline floor of the primitive pharynx between the first and second pharyngeal pouches. This will develop in the thyroid, which will migrate caudally along the thyroglossal duct (TGD). (Color version of figure is available online.)
During embryologic development, abnormal descent of the thyroid gland can occur and ectopic thyroid tissue can be found anywhere from the base of the tongue to the diaphragm. This is an important entity to recognize, as this ectopic tissue may be mistaken for neoplastic disease.2
As the TGD involutes, a small amount of residual tissue may persist distally to form the pyramidal lobe. The incidence of this lobe was found to be 55% in a postmortem study.8 It is important to identify this anatomic variant to avoid leaving residual tissue when total thyroidectomy is performed. The pyramidal lobe may branch from the right or left lobe, the isthmus, or from only one lobe with absence of the isthmus (Fig. 5A). On imaging studies, the pyramidal lobe demonstrates imaging characteristics similar to the normal thyroid gland. The shape of the lobe is variable, ranging from short and thick to long and thin (Fig. 5). The pyramidal lobe is most commonly identified in Graves’ disease.9
Lingual Thyroid The foramen cecum at the base of the tongue is the most common ectopic thyroid location, accounting for 90% of cases. Up to 10% of ectopic thyroid tissue is found in a variety of additional locations, including the sublingual space, TGD, mediastinum, heart, and esophagus.3 The origin of those locations outside the TGD pathway is probably related to the ultimobranchial bodies of the fourth pouch. This theory, however, is debated because it is not completely clear whether these cells have the potential to form follicular cells, with some authors believing that unusual locations of ectopic thyroid tissue may simply be related to aberrant migration.4 The incidence of lingual thyroid tissue in clinical studies is estimated to range between 1 in 3000 and 1 in 100,000 cases; however, postmortem studies have shown an incidence of approximately 10%.5 The underlying cause for this arrest is not completely understood. The lingual thyroid presents clinically as a firm, pinkish, painless mass covered by epithelium at the base of the tongue.6 Lingual thyroid tissue is associated with a topic thyroid gland in only 30% of cases. In the remaining 70% of cases, the lingual thyroid is the only thyroid tissue present without an associated topic gland. Ectopic thyroid tissue is usually imaged with nuclear scintigraphy, which not only demonstrates uptake in the lingual ectopic thyroid but also assesses whether there is any normal functioning tissue within the expected topic thyroid location. On magnetic resonance (MR) and com-
TGD Cyst Similar to ectopic thyroid tissue, a TGD cyst can occur anywhere from the foramen cecum to the lower midline of
Figure 2 Primitive thyroid tissue penetrates the underlying mesenchymal tissue and descends along the TGD. The duct travels just anterior to the hyoid bone and laryngeal cartilages. Ectopic thyroid tissue and TGD cyst can be present anywhere along this pathway. (Color version of figure is available online.)
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Figure 3 Magnetic resonance imaging of a patient with difficulty in swallowing demonstrates a heterogeneous legion with areas of high signal on T2-weighted imaging (arrow, A), slight hyperintensity on T1-weighted imaging (arrow, B), and postcontrast enhancement (arrow, C) in the base of the tongue. Note the lack of a normal thyroid gland in the expected location in the axial T1-weighted imaging of the neck (D). Pertechnetate scintigraphy demonstrates uptake in the base of the tongue without uptake in the thyroid bed in the neck. (E) These findings are consistent with a lingual thyroid.
Anatomy and embryology of thyroid and parathyroid
Figure 4 Axial computed tomographic (CT) images of neck at the level of the tongue base demonstrate a hyperdense lesion (A) with coarse calcification (B). Surgically proven benign adenoma in the ectopic lingual thyroid. (Images courtesy of Dr. Yoshimi Anzai, University of Washington.)
the neck. This lesion results from failure of TGD obliteration. TGD cysts are usually located in the midline, most commonly infrahyoid (25%-65%), but they may be suprahyoid (15%-50%), at the hyoid level (20%-25%) (Fig. 6), or even intralingual (3%)4,9-11 (Fig. 7). These lesions have cystic characteristics on imaging studies. If a solid component is identified, the diagnosis of associated carcinoma should be considered.
Thyroid Gland Anatomy The thyroid gland resides in the midline of the lower neck, following its descent from the base of the tongue along the TGD, as described earlier. The gland is composed of right
107 and left lobes, typically interconnected by an isthmus in the midline, lying anterolateral to the larynx and trachea at approximately the level of the second and third tracheal rings.12 The normal thyroid gland weighs approximately 30 g. It is slightly heavier in women and becomes enlarged during pregnancy.13 The thyroid gland resides within the visceral space, anterior to the prevertebral space, surrounding the trachea and lying posterior to the infrahyoid strap muscles (sternohyoid and sternothyroid) (Figs. 8-10). The thyroid gland is attached to the larynx and trachea within the visceral space and, therefore, moves with the larynx during swallowing.9 When the thyroid gland becomes enlarged, it may extend inferiorly into the superior mediastinum, commonly described as a retrosternal thyroid. The superior and inferior thyroid arteries provide blood supply to the thyroid gland. These vessels have many anastomoses providing a rich vascular supply to this gland. The inferior thyroid artery is a branch of the thyrocervical trunk that arises from the subclavian artery. It courses anterior to the vertebral artery and longus colli muscles. The superior thyroid artery is the first branch of the external carotid artery, arising just below the hyoid bone (Fig. 11). These vessels can usually be identified on MR and CT angiography. Venous drainage is in the form of a plexus that drains into the internal jugular and brachiocephalic veins. The middle and inferior cervical ganglia of the sympathetic chain provide sympathetic innervation to the thyroid gland, whereas the vagus nerve provides parasympathetic regulation. An important anatomic structure related to the thyroid gland is the recurrent laryngeal nerve (RLN). Damage to the RLN is the most common and serious complication of thyroidectomy.14 The nerve itself is usually not identified on imaging studies. However, knowledge of its course and variants is crucial. The RLN is a branch of the vagus nerve, usually arising in the upper thorax with a different course on each side of the neck. On the left, the RLN extends inferiorly, loops under the aortopulmonary window, and ascends in the left tracheoesophageal groove. On the right, the RLN loops under the right subclavian artery and ascends in the right tracheoesophageal groove. Both nerves enter the cricothyroid articulation through the fibers of the inferior pharyngeal constrictor muscle.15 Clinical information regarding the side of vocal cord paralysis is necessary, given these anatomic differences, so that the scan range can be properly protocoled (Fig. 12). The RLN may have a nonrecurrent pathway in patients with an aberrant right subclavian artery; in which case, the nerve branches from the vagus nerve and travels directly to the larynx. This is less commonly seen on the left side in patients with right aortic arch.16
Parathyroid Gland Embryology The parathyroid glands develop from the third and fourth pharyngeal pouches (Fig. 13). Usually 2 parathyroid
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Figure 5 Illustration (A) demonstrates multiple connection sites for the pyramidal lobe to the thyroid gland. Hyperdense mass with areas of hypodensity originating from the thyroid isthmus demonstrated in these postcontrast axial CT images (B-D), consistent with a residual pyramidal lobe in a patient with Graves’ disease. (Color version of figure is available online.)
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Figure 7 Round hypointense lesion without enhancement on sagittal postcontrast T1-weighted imaging (A) and hyperintense on axial T2-weighted imaging (B) in the base of the tongue, consistent with a TGD cyst.
Figure 6 Nonenhancing cystic lesion located posteriorly to the hyoid bone asymmetric to the left in these postcontrast axial (A), coronal (B), and sagittal (C) CT images, consistent with the most common location for a TGD cyst.
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Figure 9 CT normal anatomy. 1, right lobe thyroid; 2, left lobe thyroid; 3, strap muscle (sternohyoid); 4, common carotid artery; 5, vertebral artery; 6, jugular vein; 7, sternocleidomastoid muscle; 8, anterior scalene muscle; 9, longus coli muscle; 10, trachea; 11, thyroid isthmus; 12, esophagus; 13, thyroid cartilage; 14, cricoid cartilage; 15, vertebral body.
Figure 8 Magnetic resonance normal anatomy. 1, right lobe thyroid; 2, left lobe thyroid; 3, strap muscle (sternohyoid); 4, common carotid artery; 5, vertebral artery; 6, jugular vein; 7, sternocleidomastoid muscle; 8, anterior scalene muscle; 9, longus coli muscle; 10, trachea; 11, thyroid isthmus; 12, esophagus; 13, thyroid cartilage; 14, cricoid cartilage; 15, aortic arch; 16, vertebral body.
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Figure 10 Axial diagram of the neck demonstrates the anatomic location of the thyroid gland within the visceral space in the midline. The gland is located anterior to the trachea and prevertebral space and posterior to the strap muscles. The superior parathyroid glands are located in the dorsal aspect of the thyroid gland superior lobe. The carotid arteries and jugular veins are located lateral to the visceral space. (Color version of figure is available online.)
Figure 11 The thyroid gland is supplied by the superior thyroid artery (a branch of the external carotid artery; arrow) and the inferior thyroid artery (a branch of the thyrocervical trunk that branches off the subclavian arteries; arrowhead). Note also the venous drainage of the thyroid gland into the internal jugular and innominate veins bilaterally. (Color version of figure is available online.)
Figure 12 Patient presenting clinically with a left vocal cord paralysis. Axial contrast-enhanced CT image at the thyroid cartilage level (A) demonstrates enlargement of the left pyriform sinus (dot) with a thickened aryepiglottic fold (arrow), consistent with a left vocal cord paralysis. Axial (B) and coronal (C) contrast-enhanced CT images at the aortopulmonary window level demonstrates an atherosclerotic aortic aneurysm (arrows) projecting anterior into the aortopulmonary window, likely compressing the left recurrent laryngeal nerve.
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glands are located 1 cm below the lower thyroid lobe. They can be located anywhere between the angle of the mandible and the upper mediastinum (Fig. 15). The incidence of intrathyroidal parathyroid tissue is quite low, approximately 2%.9 Arterial supply to parathyroid glands is from the superior and inferior thyroid arteries that supply the superior and inferior glands, respectively. The venous drainage is usually to thyroid veins. These glands are very small, measuring approximately 6 mm in craniocaudal dimension and 3-4 mm in transverse dimension, with a flattened “oval disk” appearance. When normal in size, these are not usually identified on most imaging studies. In contrast, parathyroid adenomas and gland hyperplasia are larger and more readily identified on imaging studies.
Conclusions Figure 13 The superior parathyroid gland originates from the fourth brachial pouch along with the ultimobranchial body that will contribute to the development of part of the thyroid gland. The inferior parathyroid glands originate from the third brachial pouch along with the thymus. (Color version of figure is available online.)
glands develop on each side, but this can range from 2 to 6 glands, with some authors reporting up to 12 glands.9,17 The cranial dorsal aspect of the third pharyngeal pouch generates the inferior parathyroid glands; the thymus also arises from the third pharyngeal pouch. The relationship of the inferior parathyroid glands and the thymus explains the variable position of the inferior parathyroid glands, as the thymus has a long course of descent into the superior mediastinum and frequently presents in ectopic locations. The superior parathyroid glands originate from the fourth pharyngeal pouch, along with part of the thyroid gland, as described previously.11 These glands start developing and migrating caudally during the fifth and sixth weeks of gestation.18,19
With the advent of minimally invasive surgical techniques, radiologists are more frequently asked to evaluate pathologies that affect the thyroid and parathyroid glands and to determine their specific anatomic location. Although the anatomy of these glands may appear simple, both are subject to considerable variability, primarily related to their somewhat complex embryology. Familiarity with the spectrum of anatomic locations and imaging appearances reflecting this embryologic variation will aid the radiologist in accurate interpretation.
Parathyroid Gland Anatomy The anatomic location of the superior parathyroid glands is relatively constant due to the close relationship between these glands and the thyroid gland. They typically reside on the dorsal aspect of the upper thyroid lobes at the level of the inferior border of the cricoid cartilage. The inferior parathyroid glands have a more variable location due to their embryologic relationship to the thymus, as discussed earlier. Fifty percent of the time, the inferior parathyroid glands are located along the lateral lower pole of the thyroid gland (Fig. 14). Fifteen percent of the time, these
Figure 14 Posterior schematic of the neck demonstrates the common locations for the parathyroid glands. Note the close proximity of the recurrent laryngeal nerves and the thyroid gland. (Color version of figure is available online.)
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Figure 15 Enhancing mass (arrow) in the superior mediastinum, just anterior to the aorta and posterior to the sternum on this postcontrast axial (A), sagittal (B), and coronal (C) CT images, consistent with an ectopic parathyroid adenoma. Tc-99m sestamibi study (D) demonstrates uptake in the superior mediastinum (arrows) at 15-minutes delay, which is persistent at 90 minutes, consistent with the ectopic parathyroid adenoma.
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