Variation in the Fractional Conversion of Plasma Estrone to Estriol among Women and Men*

In the present investigation, we determined the transfer constant of conversion of plasma estrone to estriol (E3) ( [rho] E1- E3BU ) in men, postmenopausal women, and premenopausal parous and premenopausal nulliparous women. We found no significant difference in [rho] E1- E3BU among men, postmenopausal women, premenopausal parous or nulliparous women (whether the latter were studied during the midfollicular or midluteal phase of the ovarian cycle), or in women who were ingesting oral contraceptives. On the other hand, there was a striking variation in the [rho]E1- E3BU among individuals, viz. 0.02-0.27, whereas in a given individual, the variation in [rho] E1- E3BU , evaluated on separate occasions, was small. In addition, we found a negative correlation between age at first fullterm pregnancy and [rho] E1- E3BU .