RESUMO
We compared unbound (free) testosterone radioimmunoassay concentrations in plasma and saliva from men, using a direct radioimmunoassay kit involving a ligand analog of testosterone as tracer. The assay failed to reveal detectable testosterone concentrations in saliva. In plasma the free testosterone levels were about 4 times lower than those obtained by calculation or ultrafiltration methods. Moreover, unexpected similar free testosterone levels were obtained in samples comparable in their total testosterone content but distinct in their steroid binding protein content (buffered testosterone dilutions). We suspect that free testosterone levels determined with this direct radioimmunoassay probably do not reflect the true free testosterone values and conclude that their significance remains to be established.