We characterized the abnormal thyroglobulin (TG) in the thyroid and serum of a 12-yr-old girl with a large sporadic multinodular goiter first noted at age 4 yr. She developed normallyand had no clinical evidence of hypothyroidism. However, her serum T4 was less than 1.0/µg/dl, T3 was 125 ng/ dl, and TSH was 155 µU/ml. Serum PBI was 9.7 /µg/dl, and more than 90% was not extractable with butanol. The 24-h radioactive iodine uptake was 55%, not dischargeable by perchlorate. Hormone formation was tested by the administration of 131I before surgery. [131I]T4 and [131I]T3) but not 131I-labeled iodotyrosines, were present in the thyroidal venous blood. Hydrolysis of 10,000 × g supernatants from three randomly obtained samples of the goiter revealed 66–77% of the 131I asiodotyrosines, 2–4% as iodothyronines, and 10–12% as undigestable material; the MIT to DIT ratio ranged from 3.1–8.7, and the T4 to T3 ratio rangedfrom 2.3–8.3. The TG level was 2.5 mg/g in the goiter and 9.4 /µg/ml in the serum. The RIA displacement curves for the goiter and serum TG levelswere both identical to the curve produced bynormal human TG. The iodine contents of goiter and serum TG were 0.49% and 0.47% (wt/wt), respectively. The T4 to T3 ratio was lower in the goiter (Ü5) than in the serum iodoprotein (Ü45), whereas the calculation of the T4 to T3 ratio in the thyroidal secretion was less than 1. The goiter and serumTG bound normally to Concanavalin A, indicating that they contained carbohydrate. When either serum- orgoiter-soluble proteins were gel-filtered (Bio- Gel A-5m), TG immunoreactivity and stable iodine elution profiles were the same, suggesting that no significant amounts of other iodoproteinswere present in the thyroid or circulation. Both serum and goiter TG elution volumes correspondedtomol wtof approximately 9 × 104 A sedimentation rate of 10–11 S was found forboth goiter and serum TG. An abnormally low mol wt of 8.5–9.0 × 104 was determinedby sodium dodecyl sulfate-electrophoresis, in good agreement with the estimates from gel filtration studies. A single band was present on sodium dodecyl sulfate-electrophoresis regardless of whether theTG was reduced before the analysis. Thus, it is very unlikely that the low molecular weightwas due to partial hydrolysis. We conclude that the patient's goiter was due to an abnormal TG, which had a low molecular weight, was well iodinated, formed thyroid hormones poorly, and was probably partially resistant to hydrolysis.