The peculiarities of the thyroid changes depending on clinical characteristics of disease and histological structure of tumor were fixed by the studies of cathepsin-like activity in blood plasma of patients with papillary thyroid carcinoma. H-cathepsin-like activity had a tendence to more significant in­crease especially in patients with concomitant benign thyroid disease with increasing the tumor category. In a case of methastatic tumor without other concomitant thyroid pathol­ogy, the degree of an increase in enzymatic activity was sig­nificantly higher comparing to the blood of patients without tumor metastases. B-cathepsin-like activity in blood plasma of patients only with tumor carcinoma was at the level of that of healthy subjects. B-cathepsin-like activity in blood plasma had especially significant increased under tumor T1 category that was connected with nodular goiter or adenoma in a case concominant carcinoma by other thyroid pathology. In pa­tients with carcinoma that was diagnosed against a background of concomitant pathology, the insignificant increase in L-cathepsin-like activity was not dependent on clinical fea­tures of disease, while in a case of carcinoma without concomitant pathology a degree of its increase was certainly dependent on a category of tumor T, its histological structure and the presence of oxyphilic cell metaplasia. We failed to get an evidence linking the changes in cathepsine-like activity in blood with the processes of intra- or extrathyroid invasion and invasion into the lymphatic or blood vessels. The results of the studies do not provide a rationale to consider that the cathepsine-like activity in blood of patients with papillary thyroid carcinoma can be the valuable additional biochemical criterion for differential diagnosis, estimation of the severity, and other characteristics of the disease.