THE ANALOG FREE TESTOSTERONE ASSAY - ARE THE RESULTS IN MEN CLINICALLY USEFUL

Men with low testosterone concentrations are usually hypogonadal. Howe ver, because Variations in the testosterone transport protein, sex hor mone-binding globulin (SHEG), directly influence the total testosteron e concentration, confirmation of a low testosterone with a measurement of free testosterone or ''bioavailable'' testosterone (BAT) is recomm ended. In the present study, we examined the relationship of SHBG with free testosterone (Coat-A-Count assay, Diagnostic Products) and with BAT in men (n = 29) and women (n = 28) who participated in a study of the metabolic determinants of body composition. As expected, total tes tosterone was strongly positively correlated with SHBG among men (r = 0.68; P < 0.01). Although the BAT was independent of SHBG in men (r = 0.02), SHBG was an important predictor of free testosterone (r = 0.62; P < 0.01). In contrast, in women serum concentrations of total testos terone (r = -0.26; P = 0.17), free testosterone (r = -0.30; P = 0.17), and BAT (r = -0.46; P = 0.013) all tended to be lower with increasing SHBG. Free testosterone was nearly perfectly positively correlated wi th total testosterone (r = 0.97) in men, among whom free testosterone represented a relatively constant percentage of the total testosterone (0.5-0.65%), and the percentage of free testosterone was unrelated to SHBG. Thus the Coat-A-Count free testosterone concentration in men, l ike the total testosterone concentration, is determined in part by pla sma SHBG. Accordingly, androgen deficiency may be misclassified with t his assay in men with low SHBG. Moreover, the previous findings of red uced free testosterone concentrations with hypertension or hyperinsuli nemia or as a risk factor for developing type 2 diabetes, conditions i n which SHBG is reduced, may have been methodology-related.