Thyroid gland
Congenital anomalies
Thymic tissue within thyroid gland

Author: Andrey Bychkov, M.D., Ph.D. (see Authors page)

Revised: 27 November 2015, last major update November 2015

Copyright: (c) 2001-2015, PathologyOutlines.com, Inc.

PubMed Search: Thymic tissue [title] thyroid gland

Cite this page: Thymic tissue within thyroid gland. PathologyOutlines.com website. http://pathologyoutlines.com/topic/thyroidthymictissue.html. Accessed December 9th, 2016.
Definition / General
Terminology
  • Intrathyroidal thymic tissue, ectopic thymus within thyroid
Epidemiology
  • Aberrant thymic tissue may be found in the neck of up to 20% of the general population, but intrathyroid ectopic thymic tissue is more rare and is usually found incidentally (Eur J Pediatr Surg 2002;12:327)
  • Usually found in prepubertal pediatric population, rarely in adults due to age related involution
  • Detected by ultrasound in 1% of general pediatric population (Thyroid 2015;25:534); increasing use of thyroid ultrasound in children results in an increased detection of intrathyroidal thymic inclusions (Horm Res Paediatr 2011;75:258)
  • Portions of thymic tissue are present in 70% of normal thyroid glands of infants studied by serial sectioning techniques (J Anat 1976;122:77), 5% of random sections of fetal thyroid (Pediatr Pathol 1993;13:431); 0.1 - 2.6% on random sections of pediatric thyroid (Pediatr Pathol 1993;13:181, J Anat 1976;122:77)
  • 50+ morphologically verified cases have been reported, including <10 cases in adults; additional 420+ cases were verified by ultrasound
  • M = F, mean age is 6.5 years at detection
Sites
  • L > R, bilateral in 5%
  • Mid to lower portion of thyroid lobes
  • Often close to the surface
Diagrams / Tables
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Thymus descending path

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Persistent cord of thymic tissue

Pathophysiology / Etiology
  • The closely related descent of the thyroid and parathyroid glands and thymus explains their ectopic location within each other (J Ultrasound Med 2015;34:1651)
  • As most of the thymus and inferior parathyroid glands are derived from the third pharyngeal pouch, they migrate together, and maldescending thymic portions can be trapped within the thyroid (Int J Clin Exp Pathol 2014;7:6375)
  • A small and inconstant portion of thymic gland derives from the fourth branchial pouch (thymus IV, accessory thymus), together with the upper parathyroid glands and ultimobranchial body, which may become another source of thymic tissue within thyroid (Pediatr Pathol 1993;13:431)
Clinical Features
  • Rarely symptomatic and may mimic solid (rarely cystic) thyroid nodule found accidentally on US or CT / MRI
  • Most cases occur in prepubertal children, correlating with a period of maximum growth of the thymus
  • Adult cases are exceedingly rare, most likely due to age-related involution (Arch Pathol Lab Med 2001;125:842)
  • Thymic remnants are usually found in an orthotopic (normal) position or around the thyroid gland; an intrathyroidal thymus is rarely contiguous with the mediastinal thymus (Horm Res Paediatr 2011;75:258)
Diagnosis
  • US is a sensitive tool because of the unique echogenicity of thymic tissue
  • FNAC and flow cytometry may confirm thymic tissue in preoperative settings, although thyroid FNA in children is challenging
  • Most of the early cases removed surgically were diagnosed only on histopathology
Radiology Description
  • US of ectopic thymus is identical to echotexture of the orthotopic thymus:
  • MRI is similar to that of the mediastinal thymus:
    • In children, normal thymic tissue is homogeneous and of low signal intensity, though slightly higher than that of muscle on T1-weighted images
    • On T2-weighted images, the signal intensity of thymus tissue is less than that of fat, and as the child ages, the thymus becomes infiltrated with more fat, thus increasing the signal of both T1- and T2-weighted images (J Clin Ultrasound 2008;36:443)
  • RAI scan: cold nodule (Journal of Pediatric Surgery 2013;1:386)
Prognostic Factors
  • Pediatric cases rarely become cystic; most resolve at puberty and do not progress (Pediatr Radiol 2010;40:1774)
  • Extremely rare to progress to neoplasm
Case Reports
Treatment
  • US surveillance of nodule without invasive procedures is advocated in children (Horm Res Paediatr 2011;75:258)
  • Aggressive approach with surgical excision is uncommon; it is recommended that the presence of a normal mediastinal thymus be confirmed before such surgery to avoid potential immunodeficiency (Journal of Pediatric Surgery 2013;1:386)
Clinical Images
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Nodular mass

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Ectopic thymic tissue

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Sonographic images

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Intrathyroidal ectopic thymic tissue

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Longitudinal US images

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Hyperechoic nodule

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Ultrasound, left thyroid lesion

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CT scan

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RAI scan

Gross Description
  • Whitish-yellow, well demarcated solid nodule, < 1 cm
  • May be confused with normal parathyroid gland, if located superficially
  • Rarely cystic (Endocr Pathol 2015;26:45)
Gross Images
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Intrathyroidal thymus gland, AFIP

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Nodule < 1 cm

Micro Description
  • Mature lymphoid tissue with numerous Hassall corpuscles embedded within thyroid gland
  • Extent may vary from a small area of thymic tissue containing one or two Hassall bodies, to complete well differentiated glands with distinct cortex and medulla (J Anat 1976;122:77)
  • Marked adipose involution in adults (Neuroradiology 2013;55:1405)
  • Small quantities of thyroid follicular epithelial cells can be embedded in the adipose tissue of the heterotopic thymus (Korean J Pathol 2011;45:547)
  • Ultimobranchial body remnants or ectopic parathyroid can be found in the vicinity (J Anat 1976;122:77, Thyroid 2008;18:1125)
Micro Images
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Intrathyroidal thymic tissue

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Degenerated Hassall corpuscle

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Ectopic thymus and parathyroid

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Subcapsular intrathyroidal thymus (E)

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Subcapsular intrathyroid thymus, rats

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Ectopic intrathyroidal thymoma

Cytology Description
Cytology Images
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Hassall corpuscle (D)

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CK+ epithelial cells

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Lymphocytes

Positive Stains
Negative Stains
Flow Cytometry Description
  • Uniformly maturing T cells (Thyroid 2009;19:403):
    • CD4/CD8 coexpression with trailing expression of either CD4 or CD8
    • CD5+ T cells
  • No clonal B cells
Differential Diagnosis