The incidence of thyroid function changes among renal cell carcinoma (RCC) patients treated with high-dose IL-2 plus IFN-alpha ranges from 9 to 59%, independently of the administration route (i.v. or s.c.) of IL-2. Although several studies demonstrated a correlation between high-dose IL-2/IFN-alpha regimens and autoimmune thyroid disease, only very limited data are available when low doses of IL-2 plus IFN-alpha are used. We prospectively studied thyroid function in 52 patients with metastatic RCC undergoing immunotherapy with low-dose IL-2 + IFN-alpha. All patients received treatment cycles consisting of both s.c. IL-2 and i.m. IFN-alpha for 4 consecutive weeks; cycles were repeated at 4-month intervals in all patients, irrespectively of their response. Thyroid stimulating hormone (TSH), 3,3',5-triiodo-L-thyronine (T3), thyroxine (T4), human anti-thyroglobulin antibodies (hTg-Ab) and human anti-thyroid peroxidase antibodies (hTPO-Ab) were assayed in all patients before and after each of the first 3 cycles. None of the patients showed clinical signs of dysthyroidism, nor required replacement or suppressive treatment on thyroid function; specifically, no statistically significant differences were found when the median pre- and post-treatment TSH, T3, T4, hTg-Ab and hTPO-Ab levels of each cycle were compared. The median TSH values after the 3 cycles were, respectively, 1.06 [Inter Quartile Range (IQR) 0.58-1.51], 1.21 (IQR 0.58-1.51) and 1.05 micro U/ml (IQR 0.67-1.73). As for thyroid hormones, median values after each of the 3 cycles were: 1.38 (IQR 1.19-1.50), 1.46 (IQR 1.17-1.66) and 1.36 (IQR 1.16-1.46) ng/ml for T3, and 8.74 (IQR 7.26-9.45), 8.67 (IQR 7.12-9.18) and 8.40 (IQR 7.12-9.33) micro g/dl for T4. These data show that a regimen of low-dose IL-2 plus IFN-alpha does not seem to affect thyroid function, neither inducing signs or symptoms of dysthyroidism, nor by causing major biochemical changes in TSH and thyroid hormone levels, or an increase in thyroid-specific auto-antibodies.