Note: Calcification was categorized as micro-calcification, which was defined as punctate echogenic foci of ≤2 mm or less with or without acoustic shadowing; and/or as macro-calcification, defined as a hyperechoic structure (>2 mm) with acoustic shadowing.

Thyroid capsule was categorized as complete or incomplete; in the latter case, tumoral tissue extends beyond the contours of the thyroid gland and invades into adjacent structures.

Note: Usual hypoechoic halo indicates the thyroid nodule was surrounded by a regular or thin hypoechoic halo, while unusal hypoechoic halo indicates the nodule was surrounded by an irregular or thick hypoechoic halo.

Note: Cervical lymph node status was assigned as normal or abnormal (enlargement, calcification and/or cystic change). Cervical lymph nodes were considered enlarged when the aspect ratio was >0.5 without calcifications or cystic changes. Cervical lymph nodes with calcification and/or cystic change were highly suspected to be metastatic.

Note: Elasticity score is assigned according to the Asteria criteria: grade 0, the lesion contained red, green and blue areas, indicating predominantly cystic composition with a typical "mosaic" sign; grade 1, the lesion area was uniformly green; grade 2, > 50% of the lesion area was green; grade 3, 50–90% of the lesion area was blue; or grade 4, > 90% of the lesion area was blue. Nodules assigned to grades 0-2 were considered benign; those assigned to grades 3-4 were considered malignant.