Abstract
An homogeneous phase radioassay (HRA) for antithyroglobulin autoantibodies (TgAb) in serum was investigated. In this method TgAb is allowed to react with 125I-Tg in solution and the immune complexes formed are separated by precipitation with sheep anti-human gammaglobulin. HRA proved to be suitable for the screening of sera prior to thyroglobulin (Tg) radioimmunoassay; being both sensitive, and unaffected by high endogenous levels of Tg. HRA was more sensitive than either of two commercial TgAb kits; a hemagglutination assay (Wellcome Australia Ltd.) and a solid phase radioassay (CIS France). Positive responses were obtained with 4 out of 42 normal subjects (titers up to 1/10), 24 out of 31 with untreated Graves' disease (titers up to 1/10,000) and all out of 18 with Hashimoto's thyroiditis (titers up to 1/10,000). Binding of 125I-Tg was displaceable with added unlabelled Tg, but in no case could it be abolished with less than 1,000 micrograms/l. Some sera exhibited more than one class of binding site and variation in both affinity and capacity for Tg was observed. It was concluded that the use of an assay standard for the reporting of results in units of concentration is invalid, although units of TgAb activity may be used as long as the analytical method is specified. Serum levels of TgAb may also be reported in semi-quantitative terms, such as in this report where a binding titer is used, or alternatively, antigen binding capacity may be reported.
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