Thyroid gland
Congenital anomalies
Parathyroid tissue within thyroid gland

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

Revised: 4 November 2015, last major update November 2015

Copyright: (c) 2003-2015, PathologyOutlines.com, Inc.

PubMed Search: Parathyroid tissue [title] thyroid gland

Cite this page: Parathyroid tissue within thyroid gland. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/thyroidparathyroidtissue.html. Accessed December 7th, 2016.
Definition / General
  • Caused by aberrant migration of parathyroid (PT) glands during embryogenesis
  • First described by Lahey in 1926 (J Surg Oncol 1984;27:271)
  • May undergo same pathologic processes as PT glands (Mod Pathol 1989;2:652, Int Surg 1997;82:87), usually functioning adenoma or hyperplasia
  • Intrathyroidal PT glands are rare, but intrathyroidal PT tissue is not
  • A survey of the thyroids of 350 infants and children suggests that the presence of thymus and PT tissue within the thyroid is so common as to be classified as normal (J Anat 1976;122:77)
Terminology
  • Definition: intrathyroidal PT gland (true intrathyroidal PT gland) is a PT gland, normal or abnormal, situated totally within the thyroid, surrounded on all aspects by thyroid parenchyma and with no capsule
  • This entity must be clearly distinguished from subcapsular / intracapsular PT gland and those glands located in crevices in the thyroid (Ann Surg 1976;183:271, World J Surg 1987;11:110)
Epidemiology
Sites
  • Superior vs. Inferior = 3:1
  • R:L = 3:2
Diagrams / Tables
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Location of intrathyroid PT adenomas

Pathophysiology / Etiology
  • A primordium of the superior PT glands (derived from 4th branchial pouch) may become trapped within the thyroid as the lateral and medial lobes fuse, resulting in an intrathyroidal superior PT gland (Ann Surg 1976;183:271)
  • The inferior PT glands (derived from 3rd branchial pouch) are pulled by the thymus during its descent, and, because of migrating a longer distance, they have an increased chance of becoming entrapped during the fusion of the thyroid lobes (Am J Surg 1992;164:496, Am J Surg 2006;191:418)
Clinical Features
Laboratory
  • May cause high PTH serum levels, hypercalcemia or hyperphosphatemia in functioning lesions (adenoma, hyperplasia)
  • PTH immunoassay in FNA aspiration fluid shows higher level than in serum
Radiology Description
  • Detect with 99m Technetium-sestamibi scintigraphy (MIBI scan), MIBI-CT (see Video below)
  • US: the most characteristic feature of intrathyroidal PT adenoma is a hyperechoic line on the ventral surface of the PT gland (Endocr J 2011;58:989)
Case Reports
Treatment
  • Surgical excision
    • Thyroid lobectomy for a missing PT tumor is the most common approach (J R Soc Med 1981;74:49)
    • Alternatively, thyroidotomy is performed over the lower one-third of the thyroid only if (a) a lower gland is missing, (b) the ipsilateral thymus is examined and removed, and (c) 3 other glands have been found (Otolaryngol Head Neck Surg 2011;144:867)
  • Careful search for hidden subcapsular PT glands is mandatory to avoid unnecessary thyroid surgery
  • Draining and ethanol ablation of a cyst
Clinical Images
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Sonogram

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Intrathyroidal PT adenoma

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MIBI scan

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Technetium sestamibi scan

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Parathyroid carcinoma

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Parathyroid glands

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Coronal oblique slices

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Right sided neck mass

Gross Description
  • Yellowish-tan soft nodule, completely enveloped by the thyroid
  • 5 mm to 4 cm in size, rarely occupying the whole lobe; mean weight 300 - 400 mg (Surgery 2012;152:1193)
  • The corresponding normal PT gland is missing
  • Adenoma may have degenerating cystic center (intrathyroidal PT cyst)
Gross Images
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Excised tissue

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Resected thyroid

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Parathyroid adenoma

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Thyroid lobe

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Intrathyroidal PT adenoma

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Enlarged right thyroid lobe

Micro Description
  • Incidental intrathyroidal PT tissue / gland has a typical histology of PT
  • Ectopic PT gland is completely encased by thyroid parenchyma
  • Adenoma may consist of any PT cell population and repeats the composition of orthotopic PT adenoma, with a lack of fatty tissue and peripheral rim of compressed PT parenchyma
  • Carcinoma is diagnosed by the presence of invasion or metastasis
Micro Images
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Ectopic thymus and PT within thyroid

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Surrounded by thyroid parenchyma

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PT hyperplasia

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Intrathyroidal PT adenoma

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Intrathyroidal PT cystic adenoma

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Intrathyroidal PT carcinoma

Virtual Slides
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Case 1

Cytology Description
  • Monotonous cell population of chief cells appeared as discohesive naked nuclei with coarse granular chromatin mimicking lymphocytes (Surgical Pathology Clinics 2014;7:515)
  • The cytoplasmic feature unique to PT is the perinuclear oil vacuoles of chief cells, which can be seen on Diff-Quik stain in ~13% of cases (Yang: Thyroid Fine Needle Aspiration, 2013)
  • Stippled nuclear chromatin (Cytojournal 2006;3:6)
  • Abundant capillaries with attached epithelial cells corresponded to the highly vascular parenchyma (Diagn Cytopathol 1999;21:276)
  • The distinction of the different PT lesions including hyperplasia, adenoma and carcinoma cannot be made solely on the basis of cytologic features
  • PT adenoma aspirate may contain microfollicular, trabecular, or papillary arrangements, colloid-like secretions, and macrophages similar to thyroid (Yang: Thyroid Fine Needle Aspiration, 2013)
Cytology Images
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Common and uncommon FNA features

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FNA

Positive Stains
Negative Stains
Electron Microscopy Description
  • EM can identify hidden chief cells in "pure" oxyphilic adenoma, explaining PTH production
Videos



Differential Diagnosis
  • Chief cell intrathyroidal PT adenoma
  • Intrathyroidal PT chief cell hyperplasia
    • Lymphocytic thyroiditis (Cancer 2007;111:130)
    • Intrathyroidal PT adenoma should be differentiated from thyroid tumors: naked nuclei on FNA, often has an admixture of different PT cell types (chief, oxyphil and clear cells), may have a rim of compressed benign PT tissue at the periphery, PTH+ / TTF1-
  • Oncocytic / oxyphilic intrathyroidal PT adenoma
  • Water-clear cell intrathyroidal PT adenoma
    • Thyroid clear cell adenoma