Multimerization of thyroglobulin (TG) takes place extracellularly in the thyroid follicle lumen and is regarded as a mechanism to store TG at high concentrations. Human thyroglobulin (hTG) has been shown to multimerize mainly by intermolecular disulfide crosslinks. We recently noted that TG of various mammalian species contains three highly conserved thioredoxin boxes (CXXC). This sequence is known to underlie the enzymatic activity of protein disulfide isomerase (PDI). As hTG formed intermolecular disulfide bonds in the absence of other proteins depending on the redox conditions and hTG concentration, the CXXCboxes of TG might provide the structural basis for selfassisted intermolecular crosslinking. To test this hypothesis we prepared a recombinant TG fragment containing the three thioredoxin boxes. This fragment exhibited a redox activity amounting to about 10 % of the activity of PDI at redox conditions supposed to be present in the extracellular space. This activity might be supplemented by the oxidizing system of the apical cell surfaces of thyrocytes facing the follicle lumen. Indeed, incubation of hTG with peroxidase and H[2]O[2] resulted in intermolecular disulfide bridge formation. Our results suggest a combined mechanism of selfassisted and peroxidasemediated disulfide bond formation leading to the intermolecular crosslinking of lumenal hTG.