THE MEASUREMENT OF SERUM THYROGLOBULIN IN THE PRESENCE OF THYROGLOBULIN ANTIBODIES

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Abstract
In a significant proportion of patients with thyroid disorders, thyroglobulin antibodies (TgAb) invalidate double-antibody radioimmunoassays (RIA) for thyroglobulin (Tg). A modified RIA procedure for determining serum Tg in the presence of TgAb is described. Two measurements are made on each sample: ratio of Ig-bound Tg to free Tg (bound:free ratio); this is measured by the distribution of a trace quantity of 125I-Tg between bound and free fractions and concentration of free, i.e., unbound Tg. This is measured in the supernatant after precipitation of IgG-bound Tg. Then, bound Tg = B:F x (free Tg) and total Tg = (bound Tg) + (free Tg). Critical factors examined in the validation of the modified method were use of immunologically intact 125I-Tg (specific activity .ltoreq. 5 mCi/mg); equilibration of tracter Tg with endogenous serum Tg and clean separation of IgG-bound and free fractions of Tg with goat anti-human IgG. Recovery of added Tg in the presence of TgAb was 105% and interassay precision of total Tg assay was .ltoreq. 9.3%. In euthyroid subjects without TgAb, the correlation between total Tg levels by the modified and standard double-antibody method was 0.931. The correlation was much poorer in the presence of TgAb ( r = 0.752) reflecting their interference in the standard method. Reference ranges for serum total Tg levels by the modified method in euthyroid, hyper- and hypothyroid subjects with and without TgAb are detailed.

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