Development of a highly sensitive assay for serum thyroglobulin using avidin-biotin system and its clinical application

Abstract

The authors have developed a highly sensitive sandwich-type enzyme immunoassay (EIA) for serum thyroglobulin using the biotin-avidin system. The sensitivity of this EIA was 0.1 ng/ml. The intra- and inter-assay coefficient of variation was 4-15% and 4-11%, respectively. Human Tg in serum was detectable in 100% of 63 normal subjects, and the normal range was from 2.3 ng/ml to 47.4 ng/ml. The recovery rate was very good. Then we applied this EIA to the follow-up study of patients with thyroid cancer. Serum Tg levels were serially determined in 15 patients with thyroid cancer who had undergone thyroidectomy (n = 9 subtotal, n = 6 total) and were receiving thyroid hormone suppression therapy. It was found that there were three groups in regard to the relationship between serum TSH and Tg. The first group showed normal TSH and normal Tg levels (n = 4). The second group showed TSH levels that were lower than the lower limit of the assay, whereas Tg levels were within the normal limit (n = 5). The last group showed both TSH and Tg that were lower than the lower limit (n = 6). In one case in the last group, recurrence of thyroid cancer was forecast by the change of Tg levels (from below 0.1 ng/ml to 5.9 ng/ml). Therefore, it is important to suppress serum Tg and TSH levels as much as possible in order to obtain the best usefulness of serum Tg determination in the follow-up study of thyroid cancer in patients who have undergone subtotal thyroidectomy or have residual thyroid tissue. We also measured Tg levels in patients with untreated Graves' disease. Twelve patients were negative for anti-Tg antibody and 18 were positive for anti-Tg antibody. Even in the antibody positive cases, serum Tg could be detected and their levels could be calculated from the recovery studies. The mean serum Tg levels in antibody positive cases were lower than in negative cases. These studies showed that our sensitive EIA for serum Tg determination was useful in practice in patients with thyroid disease.