Skin and vaginal routes of hormone therapy are not subject to first pass metabolism, and so lack the anabolic effects that oral therapy has on liver synthesis of vitamin K-dependent clotting factors, possibly explaining why oral therapy may increase blood clot formation.While a 2018 review found that taking progesterone and estrogen together can decrease this risk, other reviews reported an increased risk of blood clots and pulmonary embolism when estrogen and progestogen were combined, particularly when treatment was started 10 years or more after menopause and when the women were older than 60 years.The risk of venous thromboembolism may be reduced with bioidentical preparations, though research on this is only preliminary. Stroke
Multiple studies suggest that the possibility of HRT related stroke is absent if therapy is started within five years of menopause, and that the association is absent or even preventive when given by non-oral routes. Ischemic stroke risk was increased during the time of intervention in the WHI, with no significant effect after the cessation of therapy and no difference in mortality at long term follow up. When oral synthetic estrogen or combined estrogen-progestogen treatment is delayed until five years from menopause, cohort studies in Swedish women have suggested an association with hemorrhagic and ischemic stroke. Another large cohort of Danish women suggested that the specific route of administration was important, finding that although oral estrogen increased risk of stroke, absorption through the skin had no impact, and vaginal estrogen actually had a decreased risk.
HRT may promote reverse cholesterol transport through induction of cholesterol ABC transporters. HRT also results in a large reduction in the pro-thrombotic lipoprotein a. Studies on cardiovascular disease with testosterone therapy have been mixed, with some suggesting no effect or a mild negative effect, though others have shown an improvement in surrogate markers such as cholesterol, triglycerides and weight. Testosterone has a positive effect on vascular endothelial function and tone with observational studies suggesting that women with lower testosterone may be at greater risk for heart disease. Available studies are limited by small sample size and study design. Low sex hormone binding globulin, which occurs with menopause, is associate with increased body mass index and risk for type 2 diabetes. Blood clots
Effects of hormone replacement therapy on venous blood clot formation and potential for pulmonary embolism may vary with different estrogen and progestogen therapies, and with different doses or method of use. Comparisons between routes of administration suggest that when estrogens are applied to the skin or vagina, there is a lower risk of blood clots, whereas when used orally, the risk of blood clots and pulmonary embolism is increased.