The Metabolic Clearance and Blood Production Rates of Estriol in Normal, Non-Pregnant Women

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Abstract
The metabolic clearance rate (MCR) and blood production rate (PB) of estriol have been measured in normal, non-pregnant women 21 to 65 years old. 6,7-3H-Estriol was administered as a pulse injection to 4 women between days 5–7 of their menstrual cycle. The disappearance of radioactivity as unconjugated estriol can be described as a function which is the sum of two exponentials. The initial component represents spread into and transfer from a space with a volume of 20.6 ± 5.4 (se) 1. The mean value for the rate constant of total removal (reversible and irreversible) was 290.2 ± 78.5 units/day of which 0.34 ± 0.06 was irreversible. The mean MCRR was 990 ± 70 1/day/m2. 4-14C-Estriol was infused at a constant rate for 3½ hours to 13 women between days 5–7 of their cycle. The mean MCR was 2,100 ± 100 1/day or 1,240 ± 40 1/day/m2. Thirteen women received a constant infusion of 4-14C-estriol between days 20–22 of their cycle. The mean MCR was 2,100 ± 100 1/day or 1,240 ± 40 1/day/m2. Thirteen women received a constant infusion of 4-14C-estriol between days 20–22 of their cycle. The mean MCR was 2,100 ± 115 1/day or 1,280 ± 65 1/day/m2. The mean values for the two phases of the cycle were not significantly different (P > 0.1). The mean value for the MCR in 4 post-menopausal women studied in similar fashion was 1,890 ± 95 1/day or 1,060 ± 35 1/day/m2. The mean concentrations of estriol were 7.0 ± 0.7 and 10.9 ± 0.8 in the follicular and luteal phases of young women, respectively. The mean PB for women in the follicular phase was 14.0 ± 1.6 μg/day and in the luteal phase was 22.7 ± 1.9 μg/day. These values were significantly different (P < 0.01). When the PB’s for the 11 women studied in both phases of the cycle were compared the luteal phase values were significantly higher 0.02 > P > 0.01) using the paired t test. The PB in the 4 post-menopausal women ranged from 5 to 22 μg/day. While there was no difference between the MCR of estriol measured in the two phases of the cycle, the PB of estriol was significantly greater in the luteal phase. Estriol probably contributes little to the overall estrogenic activity in normal, non-pregnant, pre-menopausal women but could make a more significant contribution in some post-menopausal women.