US images of TI-RADS classification. Ultrasound TI-RADS category 2 (A, mainly cystic), TI-RADS category 3 (B, no suspicious features), TI-RADS category 4a (C, solidity), TI-RADS category 4b (D, solidity and hypoechogenicity), TI-RADS category 4c (E, solidity, marked hypoechogenicity, irregular margins and taller-than-wide shape), TI-RADS category 5 (F, solidity, hypoechogenicity, irregular margins, microcalcifications and taller-than-wide shape); TI-RADS = thyroid imaging reporting and data system.