Oral combined hormone replacement therapy (HRT) with oestradiol and norethisterone increases plasma levels of prothrombin fragment 1+2 (F1+2), indicating an increase in thrombin generation, but the mechanisms underlying this increase are uncertain. The aim of this randomized, placebo-controlled study was to determine whether an increase in factor VII, a factor that combines with tissue factor to activate the extrinsic pathway, or a decrease in tissue factor pathway inhibitor (TFPI), an inhibitor of extrinsic pathway activation, may contribute to increases in thrombin generation occurring with HRT. Healthy postmenopausal women aged 50–75 years received placebo (n = 19) or oral combined HRT (n = 18) and had blood collected for measurement of factor VII coagulation activity (VIIc), activated factor VII (VIIa) and TFPI at baseline and at 6 weeks. Baseline characteristics were similar in the two groups, including age, body mass index and cholesterol levels. As reported previously, HRT increased the F1+2 concentration by 20%. Placebo had no effect on VIIc, VIIa or TFPI, but 6 weeks of combined HRT decreased VIIc [from 1.11±0.06 (mean±S.E.M.) to 1.03±0.06 i.u./ml; P < 0.03], VIIa [from 43.9; 10.8–198.3 (median; range) to 35.0; 6.3–66.8 m-units/ml; P < 0.03] and TFPI [from 81.3±6.5 to 60.4±5.5 ng/ml; P < 0.0001]. The decrease in TPFI with HRT was not correlated with the elevation in F1+2 levels. In conclusion, the increase in thrombin generation seen with HRT is not due to an effect on factor VII; in addition, while a contribution from the decrease in TFPI is possible, increased thrombin generation is not directly related to the decrease in TFPI.

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