The effect of interferon-beta (IFN-β) on thyroid function was studied in patients with chronic hepatitis who had no preexisting thyroid disease. Eleven patients (9 males and 2 females) aged 20 to 65 years, with a mean age of 47.7±13.5 years, were treated with 6 million units of IFN-β intravenously every day for 8 weeks. During IFN-β administration (4th to 8th week of treatment), both serum free thyroxine (FT4) and free triiodothyronine (FT3) concentrations decreased significantly (P<0.0005 and P<0.05, respectively): FT4, 1.37±0.17 to 1.09±0.12ng/dl, and FT3, 3.71±0.45 to .28±0.34pg/ml. On the other hand, serum TSH increased significantly from a baseline of 1.70±0.82 to 3.34±1.98μU/ml during IFN-β administration (P<0.005). Four to eight weeks after cessation of treatment, the mean serum FT4 concentration was similar to that during IFN-β administration (1.04±0.14ng/dl), but mean serum FT3 and TSH concentrations returned to pre-treatment levels (FT3, 3.57±0.42pg/ml and TSH, .60±0.84 μU/ml). Both reverse T3 and thyroglobulin were essentially unchanged. Tests for anti-thyroglobulin and anti-microsomal antibodies were negative in all the patients. These results indicate that IFN-β may inhibit thyroid function in patients without preexisting thyroid disease irrespective of humoral immune responses.