Distant and lymph node metastases of thyroid nodules with no pathological evidence of malignancy: a limitation of pathological examination.

Among thyroid nodules arising from follicular cells, benign nodular goiter is thought not to metastasize to regional or distant organs. However, we encountered five cases that were pathologically diagnosed as benign nodular goiter but showed metastasis. The prevalence of benign nodular goiter showing metastasis was 0.17% (5 of 2978 patients). On pathology, there were no detectable signs of carcinoma or follicular adenoma lesions. Two patients showed lymph node metastasis that was pathologically confirmed as metastasis of nodular goiter. One was preoperatively and another was postoperatively detected by ultrasonography. These patients also showed distant metastases that could be ablated by radioiodine. One patient preoperatively showed lung metastasis and the remaining two showed lung and bone metastases and bone metastasis postoperatively. Pathological diagnosis of thyroid nodules has limitations, and cases diagnosed as benign nodular goiter should still undergo careful follow-up.