Table 1.

The classifications and definitions of nodule features.

Features	Classifications	Definitions
Size intervals	D ≤ 0.5 cm, 0.5 < D ≤ 1.0 cm, 1.0 < D ≤ 2.0 cm, 2.0 < D ≤ 2.5 cm	Maximum diameter of the nodule
Composition	Cystic/sponge-like/mixed/solid	Cystic: the complete composition of the nodules is cystic/sponge-like: composed of multiple microcystic components, accounting for more than 50% of the nodule volume/mixed: solid and cystic components are >5%/solid nodules: solid components >95%
Echogenicity	Absence/hyperechogenicity or isoechogenicity/hypoechogenicity/very hypoechogenicity	Absence: for cystic or almost complete cystic nodules/hyperechogenicity or isoechogenicity/hypoechogenicity: reference is normal thyroid tissue next to the nodule/very hypoechogenicity: reference is adjacent to the anterior cervical muscle
Margin	Smooth/ill-defined/lobulated or irregular/extrathyroidal extension	Smooth: the boundary between the nodule and the parenchyma of the gland is clear/ill-defined: not clear/lobulated or irregular: the dividing line between nodule and surrounding thyroid parenchyma is irregular/extrathyroidal extension: the capsule of thyroid is incomplete and the continuity is interrupted
Shape	Tall/wide <1/tall/wide >1	Wider-than-tall/taller-than-wide
Echogenic foci pattern	None or large comet-tail artifacts/macrocalcification/peripheral (rim) calcifications/punctate echogenic foci	Large comet-tail artifacts: artifact depth >1 cm, present in cystic composition/coarse calcification: rear with sound shadow/peripheral calcification: calcification completely or partially encircles most of the edge of the nodule/punctate echogenic foci: no sound behind, diameter <1 cm, including the small hui tail sign in the solid composition