Thyroid & parathyroid

Benign thyroid neoplasms

Follicular adenoma

Last author update: 1 November 2014
Last staff update: 3 April 2023 (update in progress)

Copyright: 2003-2023,, Inc.

PubMed search: follicular adenoma thyroid

Sheren Younes, M.D., Ph.D.
Page views in 2022: 48,423
Page views in 2023 to date: 24,301
Cite this page: Younes S. Follicular adenoma. website. Accessed June 4th, 2023.
Definition / general
  • Benign tumor that shows evidence of follicular differentiation but lacks evidence of capsular and vascular invasion and lacks papillary carcinoma nuclear features
  • Atypical adenoma: pleomorphism, cellularity, mitotic figures or necrosis but without capsular or vascular invasion; typically considered to have benign behavior but may be precursor of anaplastic carcinoma (Hum Pathol 2003;34:666)
  • Adults, usually 20 - 50 years
  • M:F = 1:6
Clinical features
Clinical features / diagnosis:
  • Presents with long standing solitary thyroid nodule
  • Almost always solitary; if multiple, diagnose as multinodular goiter with adenomatous change
  • Suster restricts diagnosis to well encapsulated, usually solitary lesions, in which the uninvolved thyroid parenchyma does not display any features of nodular hyperplasia (Arch Pathol Lab Med 2006;130:984)
  • Patient is usually euthyroid
Radiology description
  • Usually "cold" nodule, may be "warm" but rarely "hot"
Case reports
  • Lobectomy (not enucleation)
Gross description
  • Solitary, encapsulated, variable size (1 - 10 cm)
  • Solid, fleshy, tan to light brown
  • Bulges when fresh, compresses adjacent thyroid
  • Resembles multinodular goiter due to secondary changes of hemorrhage and cystic degeneration
Gross images

Contributed by Andrey Bychkov, M.D., Ph.D., Mark R. Wick, M.D.

Encapsulated nodule

Grossing and sampling

AFIP images

Encapsulated, homogeneous tan cut surface

Bisected adenoma has fresh hemorrhage

Marked necrosis, hemorrhage and cystic change

Marked cystic degeneration

Images hosted on other servers:

Well circumscribed tumor

Well encapsulated

Central scar

Cystic and partially necrotic tumor

Microscopic (histologic) description
  • Completely enveloped by thin fibrous capsule
  • Architecturally and cytologically different from surrounding gland; surrounding thyroid tissue shows signs of compression
  • Closely packed follicles, trabeculae or solid sheets
  • Patterns:
    • Normofollicular (simple)
    • Macrofollicular (colloid): large colloid filled follicles with flattened epithelium
    • Microfollicular (fetal): small follicles, may have signet ring cell features (Am J Surg Pathol 1984;8:705)
    • Trabecular / solid (embryonal): cords / trabeculae with few follicles
  • May have focal papillary pattern
  • Cuboidal to low columnar cells, pale staining with round inconspicuous nucleoli
  • Mitoses are uncommon
  • Commonly secondary changes of hemorrhage, hemosiderin deposition, sclerosis, edema, necrosis and cystic changes
  • May have spindle cell metaplasia (Am J Clin Pathol 2002;117:199), muscular cushions (focal thickening of vessel walls) in capsule, papillary hyperplasia (Histopathology 2001;39:25), bizarre nuclei (large hyperchromatic nuclei in clusters)
  • Rarely contains mature fat (Mod Pathol 1989;2:506), signet ring cells (Acta Cytol 2004;48:87), extensive bone metaplasia (J Exp Clin Cancer Res 2001;20:443)
  • No capsular or vascular invasion after thorough sampling (at least 10 blocks), no / rare mitotic figures, no papillary nuclear features
Microscopic (histologic) images

Scroll to see all images.

Contributed by Andrey Bychkov, M.D., Ph.D., Mark R. Wick, M.D., Asmaa Gaber Abdou, M.D.

Abundant crystals of calcium oxalate

Calcium oxalate crystals

Processing artifact with distorted nuclei

Distorted nuclei due to technical / processing artifact

Tissue degeneration

Various images

Cellular follicular adenoma

Trabecular type

AFIP images

Thin and uniform fibrous capsule

Marked fibrosis and stromal hyalinization

Marked hyaline thickening of vessel walls

Marked fibrosis, hyalinization and calcium deposition

Marked vascularization

Capsular vessel with smooth muscle cells

Bizarre nuclei

Large, extremely irregular nuclei

With papillary hyperplasia

With papillary hyperplasia and adipose metaplasia

Cartilaginous metaplasia (adenochondroma)

Toxic adenoma is solitary and unencapsulated

Prominent clear cell change

Squamous metaplasia

Mucin production


A: solid,
B: trabecular,
C: microfollicular,
D: macrofollicular

Left: macrofollicular, right: solid pattern

Signet ring

Atypical adenomas:

Markedly cellular with irregular growth

Well formed follicles merge with solid pattern

Spindle cells mix with round cells

Not invasion:

Fine needle induced changes resemble invasion


Signet ring
follicular adenoma
is thyroglobulin+

Signet ring pattern, Alcian blue+

Images hosted on other servers:

Adjacent atrophic follicular epithelium

Clear cell adenoma





Encapsulated microfollicular adenoma


Spindle cell type

Cytology description
  • High cellularity, syncytial 3 dimensional arrangement, prominent nuclear crowding but no papillary nuclear features, minimal colloid
  • Cannot rule out carcinoma based on cytologic findings
  • Not all small groups of follicular cells are consistently classified as microfollicles in FNA, and some are more often classified as macrofollicles (Arch Pathol Lab Med 2006;130:148)
Cytology images

Contributed by Ayana Suzuki, C.T.


Images hosted on other servers:



FNA and H&E

Round hyperchromatic nuclei

Positive stains
Negative stains
Electron microscopy images

AFIP images

Abundant dilated endoplasmic reticulum

Microvilli project into well developed lumina

Signet ring follicular adenoma

Molecular / cytogenetics description

Solitary thyroid nodule

Thyroid: compare and contrast

Histopathology thyroid: follicular adenoma (microfollicular)

Differential diagnosis
Back to top
Image 01 Image 02