To analyze the ultrasonographic (US) findings of thyroid nodules that yielded false negative results after an initial ultrasound-guided fine needle aspiration biopsy (FNAB).

Between August 2003 and February 2006, 389 patients with 405 thyroid nodules received a repeat US-guided FNAB. We retrospectively reviewed the US findings, cytology results and postsurgical pathological results. The cytology diagnoses were classified as benign, a suspicious malignancy, a follicular neoplasm, a papillary carcinoma, and a non-diagnostic result. The US findings of the thyroid nodules were analyzed with regard to size, internal content, shape, margin, echogenecity, and calcification pattern.

Of the 405 thyroid nodules, 17 nodules were false negative. The major US findings of these nodules were a solid internal component in 16 nodules, hypoechogenicity or marked hypoechogenicity in 14 nodules, microcalcifications in 12 nodules, an ovoid to round shape in 9 nodules and a well-defined smooth margin in 9 nodules.

An repeat US-guided FNAB should be performed if the thyroid nodules have one of the malignant US features such as hypoechogenecity or marked hypoechogenecity, a microcalcification, a taller than wide shape or a well-defined spiculate margin although the cytology results indicated a benign lesion. In addition, thyroid nodules with findings of a well-defined smooth margin, ovoid to round shape, and solid internal component might also be subject to a repeat US-guided FNAB to exclude a malignancy.