A 71-yr-old man, clinically euthyroid, with a 570-g goiter causing severe mechanical neck compression underwent thyroidectomy. His total serum T4 level was 1.8 micrograms/dL, T3 was 200 ng/dL, and TSH was 35 microU/mL, and a perchlorate test was markedly abnormal. The excised thyroid tissue had normal peroxidase activity in the tyrosine iodinase and guaiacol assays. [131I]Iodide, given 24 h before surgery, was distributed in thyroglobulin isolated in vitro as follows: monoiodotyrosine, 71.6%; diiodotyrosine, 26.7%; T3, 1.05%; and T4, 0.65%. The [131I]iodide content of the whole thyroid homogenate was 59%. The goiter content of thyroglobulin was 94.7 mg/g tissue. The thyroglobulin reacted normally with antihuman thyroglobulin antiserum. Fresh goiter slices and slices from five normal human thyroid specimens were incubated with 10(-6) M KI and [131I]iodide (tracer) containing medium alone (basal), medium plus 1 mg/mL glucose oxidase (GO), and medium plus 10(-4) M NADPH and 10(-5) M vitamin K3 (NA-K3). The percentages of organic iodine in the slices, measured as protein-bound 131I, were: basal: goiter, 0.8%; normal, 6.9 +/- 1.8% (+/- SE); GO: goiter, 15.1%; normal, 17.4 +/- 3.1%; and NA-K3: goiter, 16.7%; normal, 4.6 +/- 1.14%. We conclude that an abnormal H2O2 supply may be the cause of the iodine organification defect in this goiter.