Does Hormone Therapy Cause Thyroid Cancer? Not So Fast.
A study involving approximately 5.7 million South Korean women found a modest association between menopausal hormone therapy and thyroid cancer. Women who had used hormone therapy for at least five years had a reported hazard ratio of 1.25, meaning their relative rate of diagnosis was about 25% higher than that of the comparison group. The overall incidence in the study population was approximately 2.4 thyroid cancers per 1,000 person-years.
Those findings deserve attention, but they do not show that hormone therapy causes thyroid cancer. This was an observational analysis based on screening questionnaires and insurance records. Researchers adjusted for several health and lifestyle factors, but studies of this kind cannot eliminate every meaningful difference between women who use hormones and women who do not. They also cannot fully resolve whether greater contact with the healthcare system led to more thyroid nodules and small cancers being discovered.
Earlier prospective research has not consistently found an increased thyroid cancer risk with menopausal hormone therapy. The new study adds a legitimate signal that should be investigated, ideally with clearer information about estrogen type, progesterone exposure, dose, route, timing, duration, thyroid histology, and patterns of medical surveillance. It does not justify panic or an abrupt decision to stop treatment. Hormone therapy should still be considered individually, based on symptoms, age, timing, health history, treatment goals, and known benefits and risks.
Few words in a health headline get attention as quickly as hormones and cancer.
Put them together, and the predictable result is fear. Women who are doing well on menopausal hormone therapy begin wondering whether they should stop. Women considering treatment may decide that enduring hot flashes, poor sleep, vaginal symptoms, or accelerated bone loss feels safer than taking another perceived risk.
The latest concern involves thyroid cancer.
A study presented at the Endocrine Society’s ENDO 2026 meeting found that women who had used hormone replacement therapy had a higher incidence of thyroid cancer. The association was stronger among those who reported using it for five years or longer.
That is worth studying. It is not the same as showing that hormone therapy causes thyroid cancer.
What the researchers studied
The analysis included approximately 5.7 million South Korean women aged 40 and older. The participants had completed standardized reproductive-health questionnaires while taking part in national breast and cervical cancer screening programs in 2010 or 2011.
Researchers then used the Korean National Health Insurance Service database to follow the women until a thyroid cancer diagnosis, death, or the end of the study period in 2023.
The investigators examined reproductive history, including the number of years between menarche and menopause, as well as reported hormone therapy use. Their statistical models adjusted for age, smoking, alcohol use, physical activity, body mass index, hypertension, diabetes, dyslipidemia, previous cancer, and household income.
The overall incidence of thyroid cancer was approximately 2.4 cases per 1,000 person-years.
Women reporting at least five years of hormone therapy had a hazard ratio of 1.25 compared with nonusers. In ordinary language, that represents a relative difference of about 25% in the rate of thyroid cancer diagnosis during the study period.
The study also found progressively higher thyroid cancer rates among women with longer reproductive lifespans. A reproductive span of 40 years or longer was associated with a hazard ratio of 1.30.
A 25% relative increase is not a 25% chance of cancer
This is where medical reporting often becomes confusing.
A hazard ratio of 1.25 does not mean that 25% of women taking hormone therapy developed thyroid cancer. It means the rate of diagnosis was approximately 25% higher relative to the comparison group during follow-up.
The distinction between relative and absolute risk is essential. When the starting incidence is relatively low, a meaningful relative increase can still represent a small absolute difference for an individual woman.
The publicly available study report gives the overall incidence for the entire cohort, but it does not provide enough group-specific absolute numbers to calculate a clear “additional cases per 1,000 hormone users” figure. Until those data are available, precise claims about the individual absolute increase would be premature.
This was an observational study
The study was large, but size does not turn an observational association into causation.
Researchers did not randomly assign women to hormone therapy or placebo. They looked backward at questionnaire responses, health records, and subsequent diagnoses. This allows investigators to identify patterns, but it cannot prove that hormone exposure was the factor responsible for the difference.
Women who use menopausal hormone therapy may differ from nonusers in ways that are difficult to measure completely. They may differ in education, health awareness, access to clinicians, frequency of medical appointments, reproductive history, medication use, thyroid testing, or the likelihood of receiving diagnostic imaging.
Statistical adjustments help, but they cannot account for every difference—particularly information that was never collected.
Could greater medical surveillance explain part of the finding?
Possibly. It should be treated as a plausible limitation rather than a proven explanation.
Women receiving hormone therapy generally require clinical evaluation and follow-up. Some will have more physical examinations, laboratory testing, ultrasounds, CT scans, MRI studies, or other encounters in which a thyroid nodule is noticed.
Thyroid abnormalities are frequently discovered incidentally. A neck, carotid, chest, or spinal imaging study performed for another reason may reveal a small thyroid nodule that leads to ultrasound and biopsy.
If one population has more opportunities for a thyroid cancer to be found, its recorded incidence can rise even when the underlying biology is not substantially different. This is often called detection or surveillance bias.
The ENDO 2026 report does not give us enough detail to determine how much medical surveillance differed between hormone users and nonusers. It therefore cannot tell us how much, if any, of the association was related to detection.
We also need to know what “hormone therapy” meant
Menopausal hormone therapy is not a single treatment.
It may include oral or transdermal estrogen, estrogen alone or combined with a progestogen, different doses, different treatment schedules, and a variety of synthetic or body-identical formulations. Vaginal estrogen is also clinically distinct from systemic treatment because systemic exposure is much lower with typical local dosing.
The publicly available report does not provide the product-level detail needed to determine whether the association differed according to:
- Estrogen alone versus estrogen combined with a progestogen
- Oral estrogen versus transdermal estradiol
- Systemic therapy versus local vaginal estrogen
- Different estrogen doses or treatment schedules
- Micronized progesterone versus synthetic progestins
- Age when therapy was started
- Years since menopause when therapy began
- The reason hormone therapy was prescribed
Without that information, the study cannot be applied evenly to every woman, every product, or every route of administration.
Did the study distinguish aggressive cancers from tiny incidental cancers?
The information released so far does not answer that clearly.
“Thyroid cancer” includes tumors with different histologic features and clinical behavior. Many differentiated thyroid cancers, especially small papillary tumors, have an excellent prognosis. Some are discovered through imaging rather than symptoms and may never have threatened the patient’s life.
That does not make a thyroid cancer diagnosis irrelevant. It does mean that cancer incidence alone cannot tell us whether an exposure is associated with more clinically consequential disease.
Important follow-up questions include whether hormone therapy was associated with larger tumors, more advanced disease, aggressive histology, metastatic disease, thyroid cancer mortality, or simply more diagnoses.
What did earlier research find?
The previous literature has not produced a consistent conclusion.
A 2021 meta-analysis of prospective cohort studies found no statistically significant association between menopausal hormone therapy and thyroid cancer risk in its random-effects analysis. Other studies of reproductive lifespan and hormonal factors have produced mixed results.
A separate 2025 study examined menopausal hormone therapy after treatment for thyroid cancer. Among women who used menopausal hormone therapy for more than five years, mortality did not differ significantly from that of nonusers. That study addressed prognosis rather than the initial development of thyroid cancer, so it does not cancel out the new finding. It does, however, reinforce that the relationship between menopausal hormones and thyroid cancer is not simple.
When a new observational result conflicts with or extends an inconsistent literature, the appropriate response is further investigation—not immediate certainty in either direction.
Is a biological relationship possible?
Yes. Possible is not the same as established.
Thyroid cancer occurs more often in women than in men, and thyroid tissue may respond to hormonal and reproductive influences. Laboratory research has identified estrogen-receptor signaling in some thyroid cancer cells, and investigators have long explored whether endogenous estrogen exposure contributes to the sex difference in thyroid cancer incidence.
The new study’s finding that both longer reproductive lifespan and longer hormone therapy use were associated with higher diagnosis rates gives researchers a reasonable hypothesis to pursue.
Still, biological plausibility does not establish clinical causation. Many plausible mechanisms fail to translate into meaningful disease risk when tested more rigorously.
Should women stop hormone therapy?
Not on the basis of this study alone.
Women should not abruptly discontinue prescribed hormone therapy because of a news headline. Stopping may bring back vasomotor symptoms, sleep disruption, mood symptoms, genitourinary symptoms, joint discomfort, or other problems that treatment had been controlling.
Menopausal hormone therapy remains one of the most effective treatments for bothersome hot flashes and night sweats. It can also treat genitourinary symptoms and reduce bone loss and fracture risk in appropriately selected women.
It also has recognized risks and is not appropriate for everyone. The balance depends on age, time since menopause, the symptoms being treated, personal and family history, cardiovascular and thrombotic risk, cancer history, liver health, the presence or absence of a uterus, and the specific therapy being considered.
This new thyroid cancer signal may eventually become another part of that discussion. It is not yet strong or detailed enough to replace individualized clinical judgment.
Does this mean thyroid screening is needed before or during HRT?
There is no evidence from this study that every woman using hormone therapy should begin routine thyroid ultrasound screening.
More screening is not automatically safer. Thyroid ultrasound can uncover very small nodules, many of which are benign and some of which lead to repeated imaging, biopsies, anxiety, or treatment without a clear improvement in health outcomes.
A clinician may reasonably examine the thyroid and review symptoms and risk factors as part of ordinary care. A thyroid ultrasound is generally reserved for a palpable abnormality, suspicious lymph node, compressive symptoms, clinically meaningful thyroid findings, or another specific indication.
Women with a history of childhood head or neck radiation, a strong family history of thyroid cancer, a known thyroid nodule, or certain hereditary cancer syndromes may require a different level of assessment. That decision should be based on the individual risk profile, not hormone therapy use alone.
What would better research look like?
The next studies need to move beyond the broad category of “ever used HRT.”
Researchers should examine the specific estrogen, dose, route, duration, progestogen exposure, age at initiation, and timing in relation to menopause. They should also account for thyroid evaluations, healthcare utilization, imaging exposure, preexisting nodules, thyroid disease, and the reason therapy was prescribed.
Just as importantly, studies should report absolute risk and distinguish small incidental tumors from clinically significant disease.
A replicated association with a dose-response pattern, consistent findings across populations, product-specific information, and evidence of more consequential thyroid cancer would be more persuasive than a single meeting presentation based on broad exposure categories.
Our clinical perspective
This is not a study to dismiss. Nearly 5.7 million participants and more than a decade of follow-up make the findings worthy of serious attention.
It is also not a study that can carry the conclusion implied by a frightening headline.
The honest interpretation is narrower: researchers observed a modest association between reported hormone therapy use and thyroid cancer diagnosis in a large South Korean population. The association was stronger with at least five years of use. The available data do not prove causation, do not yet clarify absolute individual risk, and do not tell us whether all forms of menopausal hormone therapy should be considered equivalent.
For a woman experiencing disruptive menopausal symptoms, treatment decisions still require a thoughtful review of the whole picture. That includes the benefits she is seeking, the therapies being considered, her age and medical history, and the risks that are already supported by stronger evidence.
Medical care becomes unstable when every new association is treated as a verdict. It becomes equally unreliable when inconvenient findings are dismissed because they complicate a preferred treatment.
This study belongs between those reactions: taken seriously, interpreted carefully, and followed by better research.
A large observational study found a modest association between menopausal hormone therapy and thyroid cancer, particularly after five years of reported use. It did not establish that hormone therapy caused the cancers. Women benefiting from prescribed hormone therapy should discuss personal risk factors and treatment goals with their clinician rather than stopping because of a headline. Routine thyroid ultrasound solely because a woman uses hormone therapy is not supported by this study.
Frequently Asked Questions
Does hormone therapy cause thyroid cancer?
The current evidence does not establish that menopausal hormone therapy causes thyroid cancer. A 2026 observational study found an association, but observational research cannot by itself prove causation.
How much higher was the risk in the new study?
Women reporting hormone therapy use for at least five years had a hazard ratio of 1.25, representing an approximately 25% higher relative rate of thyroid cancer diagnosis compared with the study’s reference group. This is not the same as a 25% absolute chance of developing thyroid cancer.
What was the overall thyroid cancer rate?
The overall incidence in the study population was approximately 2.4 cases per 1,000 person-years. The publicly available report did not provide enough group-specific absolute data to translate the relative association into a precise number of additional cases among hormone users.
Should I stop HRT because of this study?
Do not abruptly stop prescribed hormone therapy based on this study or a media headline. Review the findings with the clinician managing your treatment, particularly if you have thyroid nodules, previous head or neck radiation, a strong family history, or another known thyroid cancer risk factor.
Should women using HRT have routine thyroid ultrasounds?
This study does not establish a need for routine thyroid ultrasound screening in all hormone therapy users. Unnecessary ultrasound screening can identify small nodules that may lead to further testing without improving outcomes. Screening decisions should be individualized.
Does the study apply to transdermal estradiol and micronized progesterone?
The public report does not provide enough product-specific information to determine whether risk differed by estrogen formulation, dose, delivery route, or type of progestogen. It should not be assumed that every menopausal hormone therapy regimen carries the same association.
Related HormoneSynergy Resources
Sources
- Endocrine Society: Women’s thyroid cancer risk may be linked to reproductive lifespan and hormone therapy
- The ASCO Post: Thyroid Cancer—Reproductive Lifespan and Hormone Therapy Potentially Linked to Incidence
- Bae JM. Hormonal Replacement Therapy and Risk of Thyroid Cancer in Women: A Meta-Epidemiological Analysis of Prospective Cohort Studies
- Yuk JS, et al. The effect of menopausal hormone therapy on thyroid cancer survivors
This article was prepared for HormoneSynergy as patient education and editorial medical commentary. It interprets research presented at ENDO 2026 alongside previously published evidence. The ENDO 2026 findings were publicly available as a conference abstract and professional-society report at the time of writing; a complete peer-reviewed publication may provide additional details or modify the interpretation. This article is not individualized medical advice and should not be used to begin, discontinue, or change prescribed hormone therapy without consultation with a qualified clinician.
This article is part of the HormoneSynergy® Longevity Medicine education series covering preventive cardiology, metabolic health, hormone optimization, body composition, and advanced diagnostics for healthy aging.
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