Studies of the TSH radioreceptor assay

We have examined several variables in the reagents and procedures used in the TSH radioreceptor assay, the binding of iodinated TSH to its thyroidal receptor. We found that iodinated bovine TSH (S.A. 30 U/mg) was more effectively bound to receptor than iodinated human TSH (S.A. 7.3 U/mg). Iodination of TSH was the Bolton-Hunter acylation method apparently prevented binding to TSH receptor. Surgically removed human thyroid tissue specifically bound 10.3 +/- 1.0 (mean +/- SEM) of added [125I]TSH, but post-mortem human thyroid bound only 3.9 +/- 0.4% of [125I]TSH (p less than 0.001). Maximal binding of [125I]TSH was found at pH 5.8. Many tissue preparations contained activity, possibly due to proteases, which inactivated TSH, and inclusion of a protease inhibitor, aprotinin, significantly increased specific binding.