Preliminary clinical studies and recent in vitro investigations suggest that iodine administration may be an effective alternative in the treatment of the diffuse euthyroid goitre of iodine deficiency. Therefore a 12-month multicentre study was initiated in which 166 patients were randomly assigned to take either 150 micrograms levothyroxine day-1 (group A, n = 61), 400 micrograms iodine day-1 (group B, n = 50), or a combination of 75 micrograms levothyroxine and 200 micrograms iodine day-1 (group C, n = 55) for 8 months with follow-up examinations at 4 and 8 months as well as 4 months after cessation of treatment. Initially, thyroid volume, as determined by ultrasound, was not significantly different in the three groups. In all three groups, during treatment a significant and comparable mean decrease in goitre size was documented (-32.1% in group A, -37.3% in group B, -38.7% in group C). After cessation of treatment in group A mean thyroid volume again increased to near the baseline value (-12.0% compared to the initial investigation), while the therapeutic effect was sustained in group B (-32.5%). In group C, only a slight rebound effect was observed (-26.3% vs baseline volume). Total thyroxine (T4) increased sharply and significantly in group A from 7.8 +/- 1.9 to 10.9 +/- 2.8 micrograms dl-1 after 8 months (P less than 0.001), but only slightly, although significantly in group B (from 7.8 +/- 1.5 micrograms dl-1 to 8.9 +/- 1.6 micrograms dl-1; P less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)