High levels of urokinase-type plasminogen activator (u-PA) have been detected in malignant tissues of various organs and these are thought to be related to the local invasion and distant metastasis of carcinomas. In the present study, the expression of u-PA was immunohistochemically investigated in various thyroid tissues by using formaline-fixed, paraffin-embedded tissue sections. Normal thyroid expressed hardly any u-PA. In some hyperplasias, weak u-PA immunoreactivity was observed in small follicles and in clusters of follicular cells in the process of regenerating hyperplastic follicles. However very intense u-PA staining was observed in all cases of chronic thyroiditis in the small follicules surrounded by numerous lymphocytes. This indicates that u-PA must be strongly implicated in this disease together with various cytokines. As much as 90% of the carcinomas in our study expressed u-PA in the cytoplasm of carcinoma cells and the expression in carcinomas was much more extensive than that in benign adenomas. Although this carcinoma is known to have a generally good prognosis, our findings suggest that u-PA reflects malignancy in tumoral growth of the thyroid and modulates this growth from its early stage.

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