A stable, reproducible radioreceptorassay for thyrotropin binding inhibiting immunoglobulins (TBII)

A radioreceptorassay for thyrotropin binding inhibiting immunoglobulins (TBII) is presented. A stable reproducible method was established based on the use of thyroid glands obtained by autopsy for the preparation of a highly purified plasma membrane from human thyroid homogenate. Lower detection limit for TSH was below 5 miu/l. The TBII activity was measured in a crude IgG precipitate from serum and the results were calculated as an index value compared to normal controls, the mean of which was defined as 1.00. The binding of [125I] TSH to this membrane preparation was stable for more than 6 months. Interassay variations of the TBII index in three ranges were 1.30 +/- 0.19, 0.37 +/- 0.08 and 0.04 +/- 0.07 (mean +/- SD) over a period of 6 months. The intra-assay variations of the TBII index in three ranges were 1.30 +/- 0.08, 0.46 +/- 0.03 and 0.09 +/- 0.03 (mean +/- SD). Seventy-six of 116 patients with untreated Graves' disease had index values below the reference interval and TBII was also detected in 4 of 14 patients with multinodular goitre. The value of the TBII index in patients with untreated Graves' disease was significantly correlated to S-T3, the 4 h [131I] uptake and the 20 min [99mTc] uptake. No significant correlation between the TBII index and long-acting thyroid stimulator (LATS) measured in a bioassay was found. However, the LATS-positive patients also had positive TBII index.