Evidence for immunological differences between circulating and thyroid tissue-derived thyroglobulin in men.

The expression of six antigenic epitopes on plasma Tg and tissue-derived Tg was investigated using mouse mabs in immunoradiometric assays. The sensitivity of the assays ranged between 0.5 and 1.5 micrograms l-1. Plasma thyroglobulin of 15 normal individuals and 18 patients with metastatic, differentiated thyroid cancer was analysed. Whereas five monoclonals produced values comparable to the conventional RIA system, the monoclonal antibody Tg 158 did not detect any Tg in the normal individuals and only low levels in two of the cancer patients with the highest Tg levels. In contrast, dilutions of extracts of three different euthyroid goitre samples and three different samples of differentiated thyroid cancer revealed no difference in the antigenic expression of all six epitopes, including the Tg 158 epitope. Papain digestion of purified Tg resulted in a 76 kD fragment which showed immunoreactivity for Tg 158 as well as for Tg 11. No interference of the binding of Tg 158 by T3, T4 or DIT could be detected. We conclude that the monoclonal antibody Tg 158 detects an epitope which is present in thyroid tissue-derived Tg but not in circulating thyroglobulin. This antibody might be useful for the investigation of the secretory process of thyroglobulin from the thyrocytes into the circulation.