Both hyper- and hypothyroidism have been reported during prolonged recombinant human interferon-alpha (rhIFN alpha) therapy. To assess the short term effects of IFN alpha therapy on thyroid hormone metabolism, we measured thyroid hormone concentrations in eight healthy volunteers for 24 h after sc administration of rhIFN alpha and, on another occasion, after sc administration of saline (control study). There were no effects of rhIFN alpha on plasma T4 and free T4 or on thyroid hormone binding index. However, rhIFN alpha induced a significant decrease in the plasma concentrations of TSH (P < 0.03) and T3 (P < 0.02) compared with those in the control study, associated with an increase in rT3 concentrations (P < 0.02). IFN alpha induced a moderate increase in interleukin-6 (IL-6) concentrations (P < 0.02 vs. control study), whereas IL-1 and tumor necrosis factor concentrations remained below the detection limit in all subjects. It is concluded that IFN alpha administration induces major changes in thyroid hormone metabolism, possibly mediated in part by IL-6. The acute effects of rhIFN alpha mimic the euthyroid sick syndrome and appear to be different from the effects of chronic rhIFN alpha treatment on thyroid hormone metabolism.