Evaluation of an immunoassay for plasma sex hormone-binding globulin: comparison with steroid-binding assay under physiological and pathological conditions.

It is possible that alterations in sex hormone-binding globulin (SHBG) binding capacity are due to changes in binding kinetics rather than changes in concentration and, therefore, the immunoreactivity of SHBG may not reflect the binding activity. In this study an immunoradiometric (IRMA) assay was evaluated and the results compared with those of an established binding capacity assay. The correlation between the results of the IRMA (y) and binding assay (x) for 179 specimens was r = 0.984, y = 0.95x + 5.9. Irrespective of the method used, SHBG values in normal non-pregnant women were significantly (P less than 0.05) higher than those in normal men, hirsute women, women with polycystic ovary syndrome, hyperprolactinaemic women and obese women, and were significantly less than those in pregnant women; SHBG levels in hirsute women rose during treatment with certain anovulants and fell in genetic males during the second decade of life independent of androgen levels or activity. While being technically simpler SHBG-IRMA provides comparable results to the classical binding assay, indicating that immunoreactivity is an excellent index of binding activity.