Essentially all of the WHI's risk and benefit findings are specific to one estrogen, conjugated equine estrogens, a mixture extracted from pregnant mares' urine, and, in the combined-hormone arm, one specific synthetic progestin, medroxyprogesterone acetate.

No other estrogen and no other progestogen was studied in WHI. So every WHI-derived number is, strictly, an estimate for that specific combination, and applying it to estradiol or to micronized progesterone is an extrapolation, not a direct finding. This cuts in both directions: it means the benefits often credited to modern estradiol-based regimens are just as extrapolated from WHI as the harms usually blamed on hormones in general, since there has not been another study on this scale done since.

What to know about this answer

One clinician's honest framing, that WHI evidence has been extrapolated in both directions, cuts against the common but oversimplified takeaways on both sides: that HRT causes breast cancer, and that modern regimens are proven safe.

What the research shows, and which source each finding rests on

The WHI estrogen-plus-progestin trial randomized 8,506 women to conjugated equine estrogens 0.625 mg/day plus medroxyprogesterone acetate 2.5 mg/day; the estrogen-alone trial randomized 5,310 hysterectomized women to conjugated equine estrogens 0.625 mg/day; no other estrogen and no other progestogen was studied

B[1]B[2]

After more than 20 years of follow-up, conjugated equine estrogens alone were associated with significantly lower breast cancer incidence (238 vs 296 cases, hazard ratio 0.78) and lower breast cancer mortality (30 vs 46 deaths, hazard ratio 0.60) than placebo; conjugated equine estrogens plus medroxyprogesterone acetate were associated with higher breast cancer incidence (hazard ratio 1.28) with no significant mortality difference

B[3]

Where the real disagreement is

Each position is shown with its own sources. A position listed here is not necessarily the accepted view.

WHI's findings should be understood as specific to CEE and CEE plus MPA, not generalized to hormones

Several clinicians in the field, and the WHI investigators themselves

WHI used a synthetic progestin, medroxyprogesterone acetate, not the progestin most commonly prescribed today, and the trial's own authors said the findings cannot be applied to current formulations; this licenses saying WHI's breast cancer finding is specific to that combination, but it does not license saying modern regimens are proven safe, since that specific comparison has never been directly tested at the same scale.

B[3]

The extrapolation problem cuts both ways, including for the benefits attributed to estradiol

At least one clinician explicitly naming the symmetry

Nearly everything known about what hormone therapy does really comes from conjugated equine estrogens data, and a great deal of it has been extrapolated to estradiol, because there has not been another large-scale study of that size since; the benefits attributed to modern estradiol regimens are, strictly, just as extrapolated as the harms popularly attributed to hormones from WHI.

B[1]B[2]

What would settle it: A WHI-scale randomized trial of modern estradiol-based regimens with hard clinical endpoints, breast cancer, cardiovascular events, fracture, mortality, has never been run; smaller trials used surrogate endpoints in far smaller cohorts over shorter durations, so this gap is likely to persist.

What is really going on here

  1. Is a claim about HRT risk actually about conjugated equine estrogens plus medroxyprogesterone acetate specifically, the only combination WHI tested?
  2. Is a claim that modern estradiol-based therapy is proven safe resting on a real outcome trial, or on extrapolation from WHI's different molecules?
  3. Does the specific estrogen and progestogen I'm being offered match what was actually studied in WHI, or is it a different, more modern combination?

Questions for your doctor

  1. What specific estrogen and progestogen am I being offered, and how does that compare to what WHI actually studied?
  2. How much of what you're telling me about the risks and benefits of my specific regimen comes from direct trial evidence versus extrapolation from WHI?
  3. Given my age and time since menopause, how does the WHI population compare to my own situation?
  4. Are there more recent, smaller studies on my specific hormone combination that add useful information beyond WHI?

Questions with no answer yet

Is there any way to know whether modern estradiol-based regimens carry the same breast cancer and cardiovascular risk pattern as the conjugated equine estrogen and medroxyprogesterone acetate combination studied in the original large trial, given that no equivalent large-scale trial of modern regimens has ever been run?

Not directly studied yet

No research does not mean it does not work, and it does not mean it works. It means nobody has tested it yet.

What is known: The original large trial studied only conjugated equine estrogens and, in the combined arm, only medroxyprogesterone acetate; no trial of that scale testing modern estradiol-based regimens with hard clinical endpoints has ever been run. Smaller trials of modern regimens used surrogate endpoints in far smaller cohorts over shorter durations.

What would test it: A trial of that same scale testing modern estradiol-based regimens with hard clinical endpoints such as breast cancer, cardiovascular events, fracture, and mortality; this has never been run and, given cost and duration, may never be.

Is the absence of a large-scale trial for modern hormone regimens because the answer is already assumed to be favorable, or because no funder has stepped in to run a trial of this scale and expense since the original one?

Not directly studied yet

No research does not mean it does not work, and it does not mean it works. It means nobody has tested it yet.

What is known: One clinician in the source material explicitly names the extrapolation problem: nearly everything known about what hormone therapy does really comes from conjugated equine estrogen data, extrapolated to estradiol, because there has not been another large-scale study since. This cuts in both directions, since the benefits attributed to modern regimens are, strictly, just as extrapolated as the harms popularly attributed to hormones in general.

What would test it: The same large-scale trial named above; until it exists, both the claim that modern regimens are safer and the claim that they carry the same risks remain extrapolations rather than direct findings.

Sources

  1. 1.Risks and benefits of estrogen plus progestin in healthy postmenopausal women: principal results From the Women's Health Initiative randomized controlled trial.Rossouw JE, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women. JAMA 2002;288(3):321-333. · Randomised trial
  2. 2.Effects of conjugated equine estrogen in postmenopausal women with hysterectomy: the Women's Health Initiative randomized controlled trial.Anderson GL, et al. Effects of conjugated equine estrogen in postmenopausal women with hysterectomy. JAMA 2004;291(14):1701-1712. · Randomised trial
  3. 3.Association of Menopausal Hormone Therapy With Breast Cancer Incidence and Mortality During Long-term Follow-up of the Women's Health Initiative Randomized Clinical Trials.Chlebowski RT, et al. Association of menopausal hormone therapy with breast cancer incidence and mortality during long-term follow-up of the WHI randomized clinical trials. JAMA 2020;324(4):369-380. · Randomised trial

The letter grades the type of study behind a claim, not the answer itself.

This page is general information based on published research and public sources. It is not medical advice and not a personal assessment, and it does not replace a conversation with your doctor. Peritale is a wellness product, not a medical device, and is not intended to diagnose, treat, cure or prevent any disease. For any medical question, speak with a licensed clinician.

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