In patients with nodular goiter, radioiodine ((131)I) therapy results in a mean reduction in thyroid volume (TV) of approximately 40% after 1 yr. We have demonstrated that pretreatment with a single, low dose of recombinant human TSH (rhTSH) doubles 24-h radioactive iodine uptake (RAIU) in these patients. We have now studied the safety and efficacy of therapy with a reduced dose of (131)I after pretreatment with rhTSH. Twenty-two patients with nodular goiter received (131)I therapy, 24 h after im administration of 0.01 (n = 12) or 0.03 (n = 10) mg rhTSH. In preceding diagnostic studies using tracer doses of (131)I, 24-h RAIU without and with rhTSH pretreatment (either 0.01 or 0.03 mg) were compared. Therapeutic doses of (131)I were adjusted to the rhTSH-induced increases in 24-h RAIU and were aimed at 100 micro Ci/g thyroid tissue retained at 24 h. Pretreatment with rhTSH allowed dose reduction of (131)I therapy by a factor of 1.9 +/- 0.5 in the 0.01-mg and by a factor of 2.4 +/- 0.4 in the 0.03-mg rhTSH group (P < 0.05, 0.01 vs. 0.03 mg rhTSH). Before and 1 yr after therapy, TV and the smallest cross-sectional area of the tracheal lumen were measured with magnetic resonance imaging. During the year of follow-up, serum TSH, free T(4) (FT(4)), T(3), and TSH receptor antibodies were measured at regular intervals. TV before therapy was 143 +/- 54 ml in the 0.01-mg group and 103 +/- 44 ml in the 0.03-mg rhTSH group. One year after treatment, TV reduction was 35 +/- 14% (0.01 mg rhTSH) and 41 +/- 12% (0.03 mg rhTSH). In both groups, smallest cross-sectional area of the tracheal lumen increased significantly. In the 0.01-mg rhTSH group, serum FT(4) rose, after (131)I treatment, from 15.8 +/- 2.8 to 23.2 +/- 4.4 pM. In the 0.03-mg rhTSH group, serum FT(4) rose from 15.5 +/- 2.5 to 23.5 +/- 5.1 pM. Individual peak FT(4) levels, reached between 1 and 28 d after (131)I treatment, were above the normal range in 12 patients. TSH receptor antibodies were negative in all patients before therapy and became positive in 4 patients. Hyperthyroidism developed in 3 of these 4 patients between 23 and 25 wk after therapy. In conclusion, in patients with nodular goiter pretreatment with a single, low dose of rhTSH allowed approximately 50-60% reduction of the therapeutic dose of radioiodine without compromising the efficacy of TV reduction.