The interaction of thyrotrophin binding inhibiting immunoglobulins with high and low affinity TSH receptors

The role of high and low affinity TSH receptors in thyrotrophin binding inhibiting immunoglobulin (TBII) measurements was evaluated. By modifying binding conditions it was possible to observe separately mainly high or low affinity TSH receptors. The greatest difference between TBII positive and negative immunoglobulins was observed when binding to high affinity receptors was measured. In contrast the difference was almost completely lost when low affinity receptors were exposed. The TBII and TSH sensitivity of the radioreceptor assay did however not completely correlate, thus suggesting a difference between TBII and TSH binding sites. During normal pregnancy a high ratio of borderline positive TBII titres was observed in a routine TBII radioreceptor binding assay. Such an assay measures concomitantly both high and low affinity receptors. By using binding conditions measuring preferentially high affinity TSH receptors the TBII positivity in normal pregnancy greatly decreased from 71 to 13%. These results suggest that TBII assay sensitivity and specificity are greatly improved when high affinity TSH receptors are observed. This is especially of importance during pregnancy when false positive TBII indexes are often seen.