Relationship between antithyroglobulin autoantibodies and thyroglobulin recovery rates using different thyroglobulin concentrations in the recovery buffer

The aim of the work was to examine the relationship between thyroglobulin autoantibody (TgAb) levels and the Tg recovery rate (TgRR) using different concentrations of Tg (50, 10, 5, and 1 μg/l) in the recovery buffer. A total number of 225 serum samples from individual patients were analyzed. Samples were selected for their TgAb in 6 groups: TgAb<limit of detection (LoD; n=50), LoD<TgAb≤60 IU/ml (n=62), 60 IU/ml<TgAb≤100 IU/ml (n=30), 100 IU/ml<TgAb≤500 IU/ml (n=41), 500 IU/ml< TgAb≤1 000 IU/ml (n=13) and TgAb>1 000 IU/ml (n=28). TgAb were measured with 2 different assays (VARELISA and BRAHMS Anti-Tgn RIA). TgAb levels and the TgRR determined using the 50, 10, 5, and 1 µg/l buffers showed strong significant correlations with a Spearmans' rho of - 0.720, - 0.688, - 0.686, and - 0.356, respectively, for the VARELISA assay and - 0.670, -0.617, - 0.570, and - 0.274, respectively, for the Anti-Tgn assay (all p<0.001). TgRRs were a median of 94.8% (30.5-113.0%), 90.8% (40.6-127.6%), 90.0% (8.2-119.3%), and 89.4% (range - 43.6-121.6%) for the TgRR determined using recovery buffers with concentrations of 50, 10, 5, and 1 μg/l respectively. With decreasing Tg concentration in the recovery buffer the percentage of abnormal results increased, although the extreme increase we found in the 1 μg/l group is largely caused by a lack of analytical precision in the 73 sera with Tg levels exceeding 5 μg/l. Our results give cause for further investigation into reviving the concept of Tg-recovery measurement using 5 μg/l Tg in the recovery buffer instead of the traditional 50 μg/l.