Competitive Ligand-Binding Assay for Measurement of Sex Hormone-Binding Globulin (SHBG)

Abstract

A competitive ligand-binding assay (CLBA) based on partition of a constant amount of radiolabeled ligand, ³H-dihydrotestosterone (DHT) between sex hormone-binding globulin (SHBG) and a fixed amount of rabbit anti-DHT has been utilized to measure serum SHBG concentration. The method is convenient, sensitive, precise and reproducible and it accurately measures differences in SHBG concentration. Serum albumin and corticosteroid-binding globulin do not interfere in this assay. A consistent inverse correlation was observed between SHBG concentrations and the percent unbound E₂ in human serum. The mean (±sem) serum SHBG concentration in newborn infant boys of 0.31 ± 0.02 U/ml was not significantly different from that in newborn infant girls (0.29 ± 0.02 U/ml). The mean value of 0.22 ± 0.03 U/ml in prepubertal boys was lower than newborn value but higher than 0.12 ± 0.01 U/ml observed in men. SHBG levels of 10 cirrhotic men were not different from normal values but that of 10 hyperthyroid men was 6–7-fold higher (0.79 ± 0.06 U/ml). Normal adult women SHBG of 0.30 ± 0.02 U/ml was similar to that in newborn infants but higher than in prepubertal girls (0.17 ± 0.02 U/ml), postmenopausal women (0.20 ± 0.03 U/ml) and hirsute women (0.14 ± 0.02 U/ml); it was, however, one half that of patients taking oral contraceptives (0.58 ± 0.09 U/ml). Increase of 20-fold of plasma levels E₂ during the first trimester of pregnancy was associated with 6–7-fold increase in the mean SHBG concentration over the nonpregnant levels. Further, 5-fold increase of E₂ until term was not associated, however, with any additional increase in SHBG levels. Increase of T₃ and T₄ levels during the first 3 days of life were associated with 50% increase in the SHBG serum level (p < 0.01). High SHBG levels in hyperthyroid patients returned to normal during effective treatment with propylthiouracil. Ten hypothyroid women had a mean SHBG level which was 40% that of normal women. These studies suggest that thyroid hormones and biologically active gonadal steroids are both important regulators of circulating SHBG.