SUMMARYThe changes in serum concentrations of thyroglobulin antibodies (TgAb) were investigated in eight patients with various thyroid disorders before, during and after thyroid surgery. TgAb was measured by a radioassay using 125I‐thyroglobulin.In six patients the TgAb decreased rapidly during operation. Of these patients, four had low concentrations pre‐operatively, and the decrease in TgAb resulted in undetectable concentrations for more than 2 days. Serum thyroglobulin (Tg), measured in the specimens with undetectable TgAb, i.e. immediately following surgery, showed concentrationvstime curves resembling curves from patients with no antibody present.Furthermore, two of these patients had such a high concentration of TgAb pre‐operatively that the initial Tg release was not sufficient to remove the TgAb, resulting in detectable TgAb throughout the investigation period. Serum samples drawn during and after operation were chromatographed according to molecular size, showing measureable Tg corresponding to 19S Tg. TgAb was only measureable in fractions corresponding to 7S IgG. The expected presence of complexed Tg and TgAb could not be demonstrated, but the analytical methods might have been unable to reveal this. The remaining two patients with thyroiditis showed no decrease in TgAb. This was compatible with a low content of Tg in the thyroid measured in one of