Hormone Therapy and Blood Clots: What Women at High Risk Need to Know
By Glow Health | Menopause & Sexual Health Specialists
One of the most common reasons women are denied hormone therapy is a history of blood clots or a known risk factor for clotting. Clinicians hear "blood clot risk" and reflexively say no, often without distinguishing between different types of hormone therapy, different routes of administration, or what the current evidence actually shows.
This blanket refusal is not supported by the science. And for the women on the receiving end of it, the consequences are real. They continue to suffer from menopausal symptoms, and they are denied treatment that could protect their heart, bones, brain, and metabolic health, all because of a misapplied caution that does not hold up to scrutiny.
The distinction that changes everything is how the estrogen is delivered.
Where the fear comes from
The concern about hormone therapy and blood clots traces almost entirely to the Women's Health Initiative (WHI) study, which found that oral conjugated equine estrogen combined with synthetic progestins increased the risk of venous thromboembolism (VTE), meaning blood clots in the veins that can travel to the lungs.
This finding was real and important. But it applied specifically to oral estrogen, and it has since been applied incorrectly to all forms of hormone therapy, including transdermal estrogen, which behaves very differently in the body.
The same confusion applies to oral contraceptives. Oral contraceptives, which contain ethinyl estradiol at doses far higher than those used in hormone therapy, do increase clot risk significantly. This is clinically relevant and appropriate to consider. But concluding from this that transdermal estrogen used for menopausal hormone therapy carries the same risk is not supported by the evidence.
Why route of administration matters so much
When estrogen is taken orally, it passes through the liver before entering the bloodstream, a process called first-pass hepatic metabolism. During this process, the liver produces clotting factors that increase the risk of thrombosis. This effect is well-documented and is the primary mechanism behind the VTE risk associated with oral estrogen.
Transdermal estrogen, delivered through patches, gels, or sprays, bypasses the liver entirely. It is absorbed through the skin directly into the bloodstream, avoiding the first-pass effect and the resulting clotting factor activation. Laboratory studies confirm this: oral estrogen increases activated protein C resistance by over 100% and raises Factor VII by 43%, while transdermal estrogen produces changes so small they are clinically insignificant.
This is not a theoretical distinction. It is a mechanistic explanation for a finding that has been confirmed in multiple large studies.
What the evidence shows
The ESTHER (Estrogen and Thromboembolism Risk) study, published in Circulation in 2007, was specifically designed to investigate whether different routes of estrogen administration carried different VTE risks. It included 271 women with documented VTE and 610 matched controls.
The findings were unambiguous: oral estrogen increased VTE risk 4.2-fold. Transdermal estrogen showed no increased risk whatsoever.
This has been confirmed across multiple additional bodies of evidence:
A 2015 systematic review and meta-analysis examining 22,489 oral estrogen users and 5,671 transdermal users found a VTE relative risk of 1.63 with oral estrogen and 1.00 with transdermal estrogen. For deep vein thrombosis specifically, oral estrogen carried a relative risk of 2.09 while transdermal carried 0.99, essentially identical to no treatment.
A large 2023 study using US commercial insurance data on over 20,000 VTE cases in women aged 50 to 64 found that transdermal hormone therapy was actually associated with a lower VTE risk than no treatment, while oral hormone therapy nearly doubled the risk.
Every major menopause society in the world, including The Menopause Society, the British Menopause Society, the European Menopause Society, the Canadian Menopause Society, and the Australian Menopause Society, supports the use of transdermal estrogen even in women with elevated VTE risk.
What about women at high risk?
This is where the evidence becomes particularly important for women who have been categorically denied hormone therapy.
The ESTHER study specifically analyzed women with known genetic clotting conditions, including Factor V Leiden and the Prothrombin Gene Mutation, which independently increase VTE risk approximately fourfold. The findings were striking:
Women with Factor V Leiden or Prothrombin mutation who used oral estrogen had a 25-fold increased VTE risk
Women with the same genetic mutations who used transdermal estrogen had a 4.4-fold increased risk, which was not statistically different from their baseline risk without any hormone therapy
In other words, transdermal estrogen added no additional clot risk beyond what the genetic condition itself already conferred.
A sub-analysis of the same study looked specifically at obese women, another group considered high risk for VTE. Obesity alone carries approximately a fourfold increased clot risk. Obesity combined with oral estrogen produced a 20-fold increased risk. Obesity combined with transdermal estrogen produced a 5.4-fold risk, again not statistically different from the baseline risk of obesity alone.
For women with a prior history of VTE, a prospective cohort study by Olié et al. following over 1,000 postmenopausal women with previous blood clots found that oral estrogen was associated with a 6.4-fold increased risk of recurrence, while transdermal estrogen showed a hazard ratio of 1.0, meaning no increased risk at all.
What about progesterone?
The choice of progestogen also matters for women with clotting risk factors. The ESTHER study found that micronized progesterone was associated with a protective effect on VTE risk, with an odds ratio of 0.7. Norpregnane derivatives, which are synthetic progestins used mainly in Europe, were associated with a fourfold increased risk. Standard pregnane derivatives showed no increased risk.
This provides further support for using oral micronized progesterone as the preferred progestogen in women with clotting concerns, alongside transdermal estrogen.
Vaginal estrogen is also safe
It is worth stating clearly: vaginal estrogen is not associated with increased VTE risk and is safe for all women, including those with a history of blood clots or known clotting disorders.
A large analysis from the Women's Health Initiative Observational Study involving over 45,000 postmenopausal women found no increased VTE risk with vaginal estrogen. A 2024 nationwide nested case-control study specifically examining recurrent VTE in women with prior blood clots found no increased risk of recurrence with vaginal estradiol use. These findings are consistent with the mechanistic explanation: vaginal estrogen produces such minimal systemic absorption that it does not meaningfully affect coagulation markers.
International Menopause Society guidelines state that low-dose vaginal estrogen is not associated with increased VTE risk and can be used without restriction in women with thrombophilia or a history of VTE.
The cost of denying treatment
Refusing hormone therapy to women with clotting risk factors on the basis of outdated or misapplied evidence is not a neutral or cautious decision. It is a decision with consequences.
Untreated menopause significantly increases the risk of cardiovascular disease, osteoporotic fracture, metabolic syndrome, cognitive decline, and depression. The health burden of undertreated menopause is substantial and measurable. A blanket refusal to discuss transdermal hormone therapy with a woman who has risk factors for VTE means denying her access to treatment that could, based on current evidence, be used safely, and that could meaningfully protect her long-term health.
What this means in practice
Women are routinely told that hormone therapy is off the table because of clotting risk, and they leave that appointment believing the matter is settled. In many cases it is not. What has actually been ruled out is oral estrogen, which does carry meaningful VTE risk and appropriately warrants caution. Transdermal estrogen is a different category entirely, and collapsing these two into a single contraindication causes real harm.
Individual clinical assessment always matters, and there is no one-size-fits-all answer for women with complex clotting histories. But the starting point should be an honest review of what the evidence actually shows, not a reflexive refusal based on a misunderstanding of which type of estrogen the research applies to.
Women who have risk factors for VTE deserve a genuine clinical conversation about transdermal estrogen, what the evidence shows, and whether it is appropriate for their specific situation. If that conversation has not happened, it is worth seeking out a clinician who is equipped to have it.
How Glow Health can help
At Glow Health, we are familiar with the evidence on hormone therapy and VTE risk and we approach this conversation with the nuance it deserves. We do not reflexively deny treatment to women with clotting risk factors. We review the full picture, discuss the relevant evidence, and work with each woman individually to determine whether transdermal hormone therapy is an appropriate option for her.
If you have been told you cannot use hormone therapy because of blood clot risk and you would like a more informed second opinion, we welcome that conversation.
Keywords: hormone therapy blood clots, HRT and VTE risk, transdermal estrogen clot risk, menopause and blood clots, Factor V Leiden hormone therapy, estrogen and thrombosis, ESTHER study, hormone therapy high risk women
This post is for informational purposes only and does not constitute medical advice. Please consult a qualified healthcare clinician for personalized guidance.
