Thyroid & parathyroid

Congenital / metabolic anomalies

Parasitic nodule



Last author update: 1 December 2015
Last staff update: 31 August 2023

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PubMed Search: Parasitic nodule [title]

Andrey Bychkov, M.D., Ph.D.
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Cite this page: Bychkov A. Parasitic nodule. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/thyroidparasiticnodule.html. Accessed March 18th, 2024.
Definition / general
Terminology
  • Also called sequestered (i.e. sequestered goiter), detached or accessory thyroid nodule
  • Recommended to use "parasitic nodule" for separated thyroid nodules in lateral neck, as opposed to midline ectopic thyroid tissue along the thyrothymic tract, which is mainly a developmental abnormality (Virchows Arch 1999;434:241)
  • Lateral aberrant thyroid often represents parasitic thyroid nodule
Epidemiology
  • F:M = 4:1, median age is 51 years (range 15 to 83 years)
  • ~100 cases have been reported; the largest series was from Dr. Rosai (Lab Invest 2006;86:96A)
Sites
  • Perithyroidal, close to the gland (< 1 cm)
  • Can be located in the lateral neck from the submandibular to the retroclavicular area, the sternocleidomastoid and sternohyoid muscles (Lab Invest 2006;86:96A)
  • Rarely found in the mediastinum as part of a substernal nodular goiter (Arch Intern Med 1983;143:1015)
Pathophysiology / etiology
  • Portion of goitrous thyroid extending through the fascia may be separated by the mechanical action of neck muscles, and remains connected to the main gland by a thin fibrous strand of vascular tissue (Boston Med Surg J 1903;149:616)
  • Split from thyroid gland is due to ablation of pre-existing connection or lack of identification of connection to the main gland (Wenig: Atlas of Head and Neck Pathology, 3rd Edition, 2015)
  • Alternatively, parasitic nodule may represent concurrent hyperplastic changes in accessory thyroid tissue (N Engl J Med 1964;270:927)
  • Blood supply may be obtained from thyroid via fibrovascular pedicle, or be autonomous, acquired from the surrounding tissues (ISRN Surg 2011;2011:313626)
Clinical features
  • Palpated in the lateral neck (N Engl J Med 1964;270:927)
  • The nodule is usually an expression of nodular hyperplasia or nodular Hashimoto thyroiditis, less commonly of Graves disease (Histopathology 2006;49:107)
  • Benign condition. but some cases of metastatic thyroid carcinoma from occult primary may be initially misdiagnosed as parasitic nodules
    • Rodriguez found malignancy without evidence of tumor in the main gland in 10% of studied parasitic nodules, and suggested that parasitic nodule can originate in a primary tumor (Lab Invest 2006;86:96A), but microcarcinoma in the main thyroid cannot be excluded
Diagnosis
  • On histopathology, after exclusion of metastatic cancer
Radiology description
Radiology images

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Neck ultrasonography

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Color flow Doppler

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Tracer uptake, lateral neck

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Radiography of mediastinal mass

Case reports
Treatment
  • Usually removed surgically to rule out metastasis
Gross description
  • 0.5 - 6.5 cm nodule, separate from thyroid gland, usually single (> 80%)
  • Fibrovascular pedicle connecting to the main thyroid can be discovered after careful dissection at surgery
  • Often nodular or shows changes similar to the main thyroid
Gross images

AFIP images
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Nodular hyperplasia



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Mediastinal thyroid mass

Microscopic (histologic) description
  • Benign appearing thyroid tissue with colloid filled or hyperplastic follicles
  • Similar features are found in orthotopic gland
  • Hashimoto thyroiditis in parasitic nodule may simulate lymph node tissue
Microscopic (histologic) images

Contributed by Andrey Bychkov, M.D., Ph.D. and AFIP

2 perithyroidal lymph nodes

Specimen with signs of Hashimoto thyroiditis

Lymphoid follicles

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Patient with Hashimoto

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Hyperplastic nodule



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Parasitic nodule vs lymph node

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Thyroid follicles

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TTF1, Thyroglobulin

Cytology description
Negative stains
Molecular / cytogenetics description
Differential diagnosis
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