A sensitive radio-immunoassay for serum thyroglobulin--including a correct screening for thyroglobulin autoantibodies

A new thyroglobulin (Tg) assay employing polyethylene glycol (PEG) for separation of free and antibody bound [125I]Tg is described. By choosing a suitable PEG concentration the non-specific precipitation of free [125I]Tg was kept low while both [125I]Tg bound to rabbit anti-Tg immunoglobulins and to human Tg autoantibodies were precipitated. For all sera a 'blank' incubation with no rabbit anti-Tg immunoglobulins was run in parallel with the assay tubes. Hence any interference of Tg autoantibodies would be detected. A two-phase incubation gave a very low detection limit of 0.2 microgram/l. The inter-assay coefficient of variation was 9.4% and the intra-assay coefficient of variation was 5.6%. Dilution curves of normal sera were parallel to the standard curve and Tg standards added to normal sera were recovered quantitatively. Twelve out of 60 normal sera from the Randers area contained Tg autoantibodies. The average serum Tg in the 48 sera without antibody was 44.6 +/- 5.2 micrograms/l (SEM).