A study of over 13,000 women found no increased risk of dangerous blood clots with modern, body identical hormone treatments, including testosterone.

The research, carried out by Newson Research with Baylor College of Medicine in the US, found that natural hormone treatments were not linked to higher rates of venous thromboembolism (VTE) – a potentially life-threatening condition that includes deep vein thrombosis and pulmonary embolism (lung clots).

The research found that women using transdermal estradiol – delivered through the skin via gels or patches – alongside micronised progesterone had clot rates in line with those seen in the general population.

Over 13,000 women

The research analysed records of 13,026 perimenopausal and menopausal women at the Newson Clinic. Researchers found just seven cases of VTE were recorded, an incidence of around 0.054%. All of these women had other risk factors for their clots such as having an operation, recent long flight or having a condition that increased their risk of clot.

Women using higher doses of estradiol were also not found to have an increased risk of clot. In addition, among over 11,000 women using testosterone cream or gel, no increased risk of blood clots was found.

Synthetic verses natural hormones

Researchers say the findings challenge long-held concerns about hormone treatment and blood clots, which were based on older studies of synthetic oral hormones. These are not actually hormones, they are chemicals made to be like hormones but are all associated with an increased risk of clots.

In contrast, transdermal hormones have the same molecular structure as the hormones produced in your body so do not carry the same risks.

Despite this, patient information leaflets for hormone treatment medicines in the UK and other countries still carry broad warnings about clot risk. In addition, current guidance recommends that women at higher risk of VTE are assessed by a specialist before starting treatment.

Reassurance

While the study was observational rather than a randomised trial, researchers say it offers more real-world evidence that natural, body identical hormone treatments are unlikely to increase the risk of blood clots.

Study co-author and Balance app founder Dr Louise Newson said: ‘The findings of our research are clear: natural body identical hormones at any dose were not associated with an increased risk of clot.

‘Our results are reassuring and underline a key issue: many of the perceived risks of hormone treatments are based on outdated data from synthetic preparations. There remains widespread confusion about how these synthetic hormones, found in older forms of HRT and all hormonal contraceptives, differ from natural hormones.

‘Our findings add weight to the argument to removing the blanket clot warnings included in patient information leaflets for all natural body identical hormone products.

‘Women deserve accurate information, and above all, access to treatment that can be truly transformative for their health and wellbeing. I hope women and healthcare professionals alike will be reassured by our findings.’

Study first author Amy Neville said: ‘This study adds to a body of evidence that’s been building for over 20 years, supporting the safety of modern, body identical hormone therapies.

'But it’s completely understandable that many women and clinicians still feel cautious. Much of that concern comes from earlier studies, where the findings were widely applied beyond the treatments they looked at. Those studies used different hormone formulations (including ones derived from pregnant mares’ urine and synthetic progestogens) which are very different to the body-identical hormones more commonly used today.

‘Women deserve clear, up-to-date information so they can make informed, balanced decisions about their care. And clinicians deserve guidance that reflects how both prescribing practice and the evidence base have evolved over time. I really hope this data helps contribute to more confident, informed conversations around hormonal care.’

Changing the conversation

Study co-author Dr Mohit Khera, a US-based leading urology specialist, said the research ‘shifts the conversation away from outdated data’.

‘Historically, concerns about blood clots have made both patients and clinicians hesitant to use hormone replacement therapy,’ he added.

‘What our study shows, in a large real-world cohort of over 13,000 women, is that modern, body-identical hormone therapy – specifically transdermal estradiol with micronised progesterone – does not appear to increase the risk of venous thromboembolism above baseline.

‘This is important because it helps shift the conversation away from outdated data based on older formulations and toward evidence that reflects how we actually practice today. Ultimately, it supports a safer, more individualised approach to treating menopausal symptoms and improving long-term health for women.’

Prevent suffering

Maggie Honey, who has antiphospholipid syndrome – a condition where the immune system mistakenly produces antibodies that make the blood more likely to clot than normal – welcomed the research findings.

She said: ‘I’m hopeful evidence like this will prevent unnecessary anxiety and suffering for women like me, with clotting disorders, who have been told HRT is a “no-no”.'

The full paper, published in the journal Translational Andrology Urology, can be accessed here.