Ultrasound is the first-line imaging modality for assessment of thyroid nodules found on clinical examination or incidentally on another imaging modality. This article is an overview of ultrasonographic features of thyroid nodules, which are used to determine the need for biopsy with fine needle aspiration (FNA).

Following concerns regarding variation in practice and overdiagnosis of clinically insignificant malignancy several professional societies have published formal assessment criteria to determine the need for FNA which are covered in separate articles.

Radiographic features

Ultrasound

Calcification

Although calcification can be seen in both benign and malignant processes, it is the ultrasound feature most closely associated with malignancy ¹.

• microcalcifications

  • punctate echogenic foci without posterior shadowing

    • often might not actually represent calcifications ⁶

  • ​most specific finding associated with malignancy (~95%) ²

  • associated with papillary thyroid carcinoma

  • colloid (in benign colloid nodules) shows comet tail artifact (not ring-down); if an echogenic focus is not definitely colloid, biopsy is warranted

• coarse calcifications

  • ​can be seen in both benign and malignant nodules

  • associated with both papillary thyroid carcinoma and medullary thyroid carcinoma

• peripheral rim calcification

  • ​​can be seen in both benign and malignant nodules

Echogenicity

• hypoechoic solid nodule

  • most papillary thyroid carcinomas

  • nearly all medullary thyroid carcinomas ³

  • benign nodules can be hypoechoic

  • if no other malignant features (e.g. calcifications) then hypoechoic nodules are typically biopsied after reaching size criteria

• isoechoic solid nodule: 25% (follicular and medullary)

• hyperechoic solid nodule: 5% chance of being malignant

• large cystic component favors a benign entity although a significant proportion of papillary carcinomas will have a cystic component

• while a halo around a well-marginated hypoechoic or isoechoic nodule is typical of a follicular adenoma ³, it is absent in >50% of benign nodules ²; what is more, up to 24% of papillary thyroid carcinomas may have a halo, be it complete or incomplete ²

Color Doppler

• intranodular flow usually malignant

• lymph nodes with increased color Doppler flow are suspicious

Other

• invasion of local structures favors anaplastic thyroid carcinoma and thyroid lymphoma

• shadowing around the edges of a nodule (edge refraction shadow) are associated with papillary thyroid carcinoma ³

• a nodule taller than it is wide is suspicious for malignancy ⁴

• irregular margins are suspicious for malignancy ⁴

Lymph nodes

• enlarged regional lymph nodes are suspicious for thyroid malignancy, especially papillary thyroid carcinoma

• microcalcifications in regional lymph nodes are highly suspicious

• lymph nodes with cystic change are highly suspicious

• loss of normal fatty hilum, irregular node appearance

• increased color Doppler flow is suspicious

• no threshold criteria for lymph node biopsy

  • biopsy if suspicious features

  • consider biopsy if >8 mm

Sonographic features favoring a benign nodule

• large cystic component

• hyperechoic solid

• comet tail artifact

• spongiform appearance / sponge-like appearance ^7,8

Sonographic features favoring a malignant nodule

• hypoechoic solid

• presence of microcalcifications: almost always warrants biopsy

• local invasion of surrounding structures

• taller than it is wide

• large size: the cutoff is often taken as 10 mm to warrant biopsy

• suspicious neck lymph nodes suggesting metastatic disease

• intranodular blood flow

Differential diagnosis

The differential for a suspicious nodule includes benign nodules such as adenomatoid nodules, follicular adenoma, and Hashimoto thyroiditis. Parathyroid adenomas are also confounding nodules.

No sonographic features are 100% sensitive or specific (although lymphadenopathy with microcalcifications is 100% specific). A suspicious nodule (and lymph node, if applicable) should undergo biopsy with fine needle aspiration. Multiple criteria exist from societies in different subspecialties and biopsy thresholds vary among institutions. See below for individual systems.

See also

• ​American Thyroid Association (ATA) guidelines

• British Thyroid Association (BTA) U classification

• McGill Thyroid Nodule Score

• Society of Radiologists in Ultrasound (SRU) guidelines

• TI-RADS

  • American College of Radiology: ACR TI-RADS[+]

  • European Thyroid Association: EU-TIRADS

  • Korean Society of Thyroid Radiology: K-TIRADS