Objective  To investigate the sonographic characteristics of secondary thyroid cancer.

  Methods  The clinical records of 10 patients with histologically proven metastasis to thyroid who were treated in Peking Union Medical College Hospital between April 1988 and July 2013 were retrospectively reviewed. The ultrasound images and reports were reviewed, and the sonographic characteristics of the metastatic thyroid cancer were summarized.

  Results  In 1 patient with lung cancer and 1 with esophageal cancer, the metastatic lesions were presented as enlarged thyroid gland and uneven diffuse echo; in the remaining 8 patients, thyroid nodules were found. In the 8 lesions, 5 were multiple and 3 were solitary; right lobe was involved in 2 cases, left lobe in 1 case, and both lobes in 5 cases; the maximal size of the metastatic nodule averaged 3.8 cm (ranged between 0.6 and 6.6 cm); 5 were ill-defined and 3 were well-defined; 7 were irregular and 1 wasn't reported; 7 were hypoechoic and 1 was hyperechoic; 6 were solid and 2 were solid and cystic; calcification was observed in 6 cases; rich blood flow was detected in 4 cases, blood flow was detected in internal and peripheral parts of the lesion in 1 case, few bloodflow was seen in peripheral part of the lesion in 1 case, and no blood flow in 1 case. The sonographic characteristics of the secondary thyroid cancer were similar to those of the primary tumor.

  Conclusions  The sonographic characteristics of secondary thyroid cancer are diverse, among which the thyroid nodule is a common manifestation and uneven diffuse echo can also be observed. The sonographic features that help to identify secondary thyroid cancer include multiple and large lesions, with ill-defined and irregular borders; the mass is solid, hypoechoic, and calcified, with rich blood flow. Ultrasound helps to select suspicious nodule for fine needle aspiration or surgery.