The aim of this study was to find reasons for the interference of sex-hormone binding globulin (SHBG) in direct radioimmunoassays (RIAs) for testosterone and estradiol. 125I-labeled testosterone and estradiol were compared with 3H-labeled testosterone and estradiol for their affinity to SHBG. The 3H-labeled steroids bound far more avidly to SHBG than did the corresponding 125I-labeled steroids. The SHBG concentration in the standard matrix of two kits was significant. Finally, 3H-labeled steroids were compared with 125I-labeled steroids for their affinity to the antibody supplied with the kits. The 125I-labeled steroids bound more avidly than did the 3H-labeled steroids to the corresponding antibodies. Direct testosterone and estradiol results would be influenced according to the SHBG in the sample, as observed.
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Masters et al. (1989) studied this question.