In 1987, Smith et al. reported the usefulness of FNA for the diagnosis and management of secondary thyroid cancers in a medicine journal. Over a six-year period, FNA diagnosed metastasis in 14 patients with known primary and established the primary in five patients without prior history of nonthyroidal cancer. Breast, kidney, and lung were the most frequent primary for carcinoma in this study. The authors concluded that FNA was able to direct appropriate surgical or conservative management without adversely affecting survival among patients. The usefulness of thyroid FNA in diagosing secondary thyroid cancer was also reported in a cytology journal in the same year.

Secondary thyroid cancers may occur as a result of direct extension from nearby structures or metastasis via the blood-stream from distant sites. Direct invasion from esophagus and head and neck tumors is far more common than hematogeneous metastasis. Typically, patients present with thyroid nodule, hoarseness from involvement of recurrent laryngeal nerves, dyspnea, dysphagia, and neck pain.

Low incidence in clinical setting

The incidence of secondary thyroid tumors in autopsy studies is up to 25% of patients with widely disseminated cancer, but much lower in clinical studies. Over a period of 36 years in the Mayo Clinic, only 30 cases (0.26% of thyroid cancers) diagnosed in surgical pathology were distant metastasis. The origin of secondary thyroid cancers reflects the incidence of cancers in different parts of the world. In one study from Hong Kong, 43% of secondary thyroid cancers are from lung, 8.8% are from breast, 7.7% from stomach, and 15% are lymphoma. In a series from India, the most common secondary thyroid tumors were direct extension from larynx (9 out of 24 cases, representing 0.12% of all thyroid FNA performed over 20 years). In a study from Korea, over a period of six years, 14 patients had secondary thyroid cancer diagnosed by FNA, including five from breast, three each from colon, kidney, and lung. In the largest study of 43 cases of secondary thyroid carcinomas from the United States, kidney was the most frequent primary (33%), followed by lung (16%), breast (16%), esophagus (9%), and uterus (7%).