BTA ultrasound "U" classification of thyroid nodules

Last revised by Henry Atkinson on 16 Oct 2023

The ultrasound "U" classification of thyroid nodules has been developed by the British Thyroid Association (BTA) as part of their 2014 guidelines on the management of thyroid cancer 1.

It allows for the stratification of thyroid nodules as benign, suspicious or malignant based on ultrasound appearances termed U1-U5. This is used to aid further investigation and management.


U1 (normal)
  • no nodules

U2 (benign)
  • hyperechoic or isoechoic with a halo

  • cystic change with ring-down artifact (colloid)

  • microcystic or spongiform appearance

  • peripheral egg-shell calcification

  • peripheral vascularity

U3 (indeterminate)
  • solid homogenous markedly hyperechoic nodule with halo (follicular lesions)

  • hypoechoic with equivocal echogenic foci or cystic change

  • mixed or central vascularity

U4 (suspicious)
  • solid hypoechoic (compared with thyroid)

  • solid very hypoechoic (compared with strap muscles)

  • hypoechoic with disrupted peripheral calcification

  • lobulated outline

U5 (malignant)
  • solid hypoechoic with a lobulated or irregular outline and microcalcification

  • solid hypoechoic with a lobulated or irregular outline and globular calcification

  • intranodular vascularity

  • taller than wide axially (AP>ML)

  • characteristic associated lymphadenopathy

Size does not have a formal role in this grading system, however the guidelines do note that in nodules under 1cm which display suspicious features but have no evidence of extra-thyroidal disease or a high risk clinical history, clinical judgment and MDT discussion should be used to decide whether to perform fine needle aspiration (FNA) in order to avoid unnecessary investigation of clinically insignificant papillary microcarcinoma.

Further investigation and management

U2 nodules do not require FNA in the absence of concerning clinical features. U3, U4 or U5 nodules require FNA with further management based primarily on resultant cytology.

The guidelines incorporate the use of the Royal College of Pathologists guidance on reporting of thyroid cytology specimens which grades specimens from thy1-thy5 based on likelihood of malignancy.

The BTA guidelines recommend repeat FNA in instances of thy1 (non-diagnostic), thy3a/thy3f (neoplasm possible) or thy4 (suspicious) results. A thy2 (non-neoplastic) result should prompt review of clinical and radiological background with repeat FNA only if high suspicion and a thy5 result (malignant) should proceed directly to further management. There is no defined role for imaging follow-up.

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