Thyroid gland
Congenital anomalies
Parasitic nodule

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

Revised: 5 July 2016, last major update December 2015

Copyright: (c) 2003-2016, PathologyOutlines.com, Inc.

PubMed Search: Parasitic nodule [title]
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 2015, 3rd Edition)
  • 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
Case Reports
Treatment
  • Usually removed surgically to rule out metastasis
Clinical Images

Images hosted on Other servers:

Missing Image

Neck ultrasonography

Missing Image

Color flow Doppler

Missing Image

Tracer uptake, lateral neck

Missing Image Missing Image Missing Image

Radiography of mediastinal mass

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

Images hosted on PathOut server:

Missing Image

Nodular hyperplasia



Images hosted on Other servers:

Missing Image

Mediastinal thyroid mass

Micro 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
Micro Images

Images hosted on PathOut server:

Missing Image Missing Image

Patient with Hashimoto

Missing Image

Hyperplastic nodule



Images hosted on Other servers:

Missing Image

Parasitic nodule vs lymph node

Missing Image Missing Image

Nodule with Hashimoto signs

Missing Image

Not a lymph node

Missing Image

Thyroid follicles

Missing Image

TTF1, Thyroglobulin

Virtual Slides
Missing Image

Case 4, parasitic sequestered nodule

Cytology Description
Negative Stains
Molecular / Cytogenetics Description
Differential Diagnosis