The use of 131I in the treatment of multinodular goiters (MNG) is well established. We evaluated the effect of 30 microCi 131I (1.11 GBq) in 18 patients with MNG with the aid of two injections of 0.1 mg recombinant human TSH (rhTSH), given on d 1 and 2. A dose of 30 microCi 131I was given on d 3. TSH, T3, free T4, and thyroglobulin were measured on d 1, 2, 3, 5, 10, 30, 60, 90, and 180, and antithyroid antibodies were measured on d 1, 30, 90, and 180. Twenty-four-hour 131I uptake measured 1-3 months before rhTSH increased from 12.3 +/- 6.2 to 53.5 +/- 10.9% (P < 0.0001), free T4 from 1.3 +/- 0.2 to peak 3.2 +/- 1.1 ng/dl levels (P < 0.0001), T3 from 113.9 +/- 35.0 to peak 332.2 +/- 123.0 ng/dl levels (P < 0.0001), TSH from 0.76 +/- 0.71 to peak 18.9 +/- 5. 3 mU/liter levels (P < 0.0001), and thyroglobulin from 280.9 +/- 370.0 to peak 1838.5 +/- 1360.7 ng/dl levels (P = 0.001). Painful thyroiditis (33%) and mild thyrotoxicosis (39%) constituted minor side effects. There were no changes in echocardiographic parameters, done before and after rhTSH administration, on d 3. Hypothyroidism developed in 65%. Mean goiter size, measured by computed tomography, decreased from 97.9 +/- 45.4 to 65.5 +/- 47.3 ml (P < 0.0001; reduction: 39 +/- 19%) after 6 months. We conclude that rhTSH is a safe and efficient therapeutic tool in the treatment of MNG allowing the use of outpatient therapeutic 131I doses.