NON-PROTEIN-BOUND ESTROGENS IN PLASMA AND URINARY-EXCRETION OF UNCONJUGATED ESTROGENS IN NON-PREGNANT WOMEN

The concentrations of oestrone and oestradiol-17β in peripheral plasma and the urinary excretion of unconjugated oestrone and unconjugated oestradiol-17β were measured by radioimmunoassay in 31 matched samples from seven young women. The concentrations of oestrone and oestradiol-17β not bound to protein in the plasma samples were measured following equilibrium dialysis. The urinary excretion of unconjugated oestrone (0.70±0.34 nmol/24 h, mean ±S.D., n = 28) was found to be significantly, but poorly, correlated with the concentration of non-protein-bound oestrone in plasma (10.2±3.8 pmol/1) (r=0.44, P<0.05). Similarly, the urinary excretion of unconjugated oestradiol-17β (0.29±0.16 nmol/24 h, n = 30) was found to be significantly, but still rather poorly correlated with the concentration of non-protein-bound oestradiol-17β plasma (7.4±5.3 pmol/l) (r = 0.58, P<0.001). Since the calculated proportions of oestrone and oestradiol-17β in plasma not bound to protein (3.4±0.3% and 1.7±0.2% respectively) remained fairly constant (coefficient of variation 9 and 10% respectively), measurement of oestrone or oestradiol-17β in plasma provided a better guide to the biologically available (non-protein-bound) hormone than did measurement of urinary unconjugated oestrogen. The mean renal clearance of both non-protein-bound oestrone and non-protein-bound oestradiol-17β (50±21 and 36±23 ml/min) was less than that of creatinine (114±31 ml/min) indicating absorption and/or metabolism of each hormone by the kidney tubule.