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Calcitonin (CT) is a sensitive marker for medullary thyroid carcinoma. Normalization of early postoperative CT level is a favorable prognostic factor. The aim of this study was to establish the prognostic value of CT-level kinetics by preoperative tumor stage and postoperative elimination rate. Blood serum from 22 medullary thyroid carcinoma (MTC) patients without remote metastases was assayed. A commercial RIA DSL-1200 test was used to assay CT levels. Basal CT< or =42 pg/ml and stimulated CT< or =120 pg/ml were considered normal. The patients were divided into three groups according to the intensity of the postoperative CT level reduction in blood serum. Group 1 comprised patients who showed basal CT normalization within the first 2-3 days after surgery. Group 2 included those patients with slow (from 2 to 4 weeks) CT-level normalization. Group 3 included patients with CT levels that reduced within 14 days, but subsequently increased. Preoperative basal CT varies from 216 to 1654 pg/ml and depends on tumor-node-metastasis (TNM) stage. In seven patients, no basal CT decrease to normal values was observed; in five of these patients, disease recurrence was detected 2-6 months after surgery. In the group with slowly decreasing CT levels, no strong correlation between preoperative CT level and the postoperative time to normalization of basal and peak CT could be established; this may be due to the small number of patients. Our study showed that preoperative CT level depended on the disease stage. Postoperative CT elimination rate is independent of preoperative CT level. Postoperative increase in the basal or stimulated CT level is an unfavorable prognostic factor, implying disease recurrence.