The hypothyroid state or nonthyroidal illness is often observed in patients with recurrent cancer. High levels of cytokines are frequently observed in critically ill patients. Recent studies have shown that interleukin (IL)-6 may be a cause of nonthyroidal illness. We reported the relationship between thyroid function and the prognosis of the patients with recurrent breast cancer. In this study, we measured the serum level of cytokines (IL-2, IL-6, IL-8) and thyroid function (free T3, free T4, and thyrotropin (TSH)) in 38 patients with recurrent breast cancer. All patients had received three or more different courses of therapy before they were entered the study. The patients were divided into three groups according to their response to therapy. There were 16 partial response (PR), 10 no change (NC) and 11 progressive disease (PD) patients. They did not receive any medication that influenced the thyroid hormone level other than medication for cancer. The IL-2 level was under the detectable limit in all groups. No abnormal levels of cytokines were observed in the PR group. IL-6 and IL-8 levels in the PD group were significantly higher than that in the NC group (p < 0.05). Significant negative correlation was observed between IL-6 and thyroid hormones (free T3, free T4). Patients whose IL-6 level was 20 pg/ml or more died within four months after the beginning of the treatment. We concluded that IL-6 may lead to a hypothyroid state in patients with recurrent breast cancer. A high level of IL-6 and IL-8 means the confusion of the defense system in hosts. Therefore, these cytokines will be predictive indicators of the therapeutic response and the prognosis of the patients with recurrent breast cancer.