Importance of the detection method for thyroglobulin antibodies for the validity of thyroglobulin measurements in sera from patients with Graves disease.

BACKGROUND The use of recovery tests has been proposed to disclose interferences from anti-thyroglobulin antibodies (TgAbs) in thyroglobulin (Tg) assays. We studied the value of a recovery test in Tg measurement by a new commercial IRMA. METHODS Blood samples were collected from 153 patients with untreated Graves disease. Tg and TgAbs were measured by IRMA and RIA, respectively (Dynotest Tg-plus and Dynotest anti-Tgn; Brahms Diagnostica). The recoveries of added amounts of Tg were calculated for each serum. RESULTS TgAbs were detected in 72 of the 153 patients (47%). The recovery test results for the 81 TgAb-negative sera (median, 101%; range, 80-115%) were identical to the results for the 91 controls (median, 102%; range, 80-124%). By contrast, significantly lower recovery test results were observed for the 72 TgAb-positive sera (median, 79%; range, 60-103%; Z = -8.363; P <0.0001). In the 34 of the 72 TgAb-positive sera with a normal recovery test, Tg concentrations were significantly lower (median Tg, 13.6 microg/L; range, 1.1-360 microg/L) than those measured in the TgAb-negative sera (median, 107 microg/L; range, 1.2-700 microg/L; Z = -3.797; P <0.0001). CONCLUSIONS Tg values were decreased in TgAb-positive sera even when the results of the recovery tests were normal. This test should not be used alone to determine the validity of a serum Tg measurement in Graves disease.