What Causes Hot Flashes? The Science Behind Vasomotor Symptoms
Dr. Karla Loken is a board-certified OB/GYN with over two decades of clinical experience and a distinguished career driving innovation in women's health across the pharmaceutical, diagnostics, and medical device industries. Through her consultancy, Loken & Associates, Dr. Loken provides medical affairs expertise and fractional Chief Medical Officer (CMO) services, helping early-stage companies bridge the gap between scientific innovation and industry application. She serves as Chief Medical Officer to MenoHealth. · 21 min read · Sep 2026
Medically reviewed by Karla F. Loken, DO, FACOOG on September 20, 2026
What causes hot flashes? Loss of estrogen activates KNDy neurons in the hypothalamus, narrowing your thermoneutral zone. The full science, explained.
It's crazy to me that there's this really absolute cultural silence about menopause. Most people have a general concept of puberty. When a woman doesn't know what to expect when she's no longer expecting a period, she doesn't know what's happening to her body.
If you’ve ever wondered what causes hot flashes — what is actually happening inside your body in the thirty seconds between feeling fine and feeling like someone turned the thermostat up by ten degrees — the answer has changed substantially in the last decade. Older explanations blamed “low estrogen” and stopped there. The newer answer is more specific, more interesting, and matters for treatment.
This guide walks through the neuroscience of vasomotor symptoms, the role of a small cluster of neurons in your hypothalamus called KNDy neurons, and why a brand-new class of medication — neurokinin 3 receptor antagonists — works on the underlying mechanism rather than the downstream symptom. It’s the science your clinician should be using to frame what you’re experiencing.
Key takeaways
Hot flashes are a thermoregulatory event driven by a narrowed core-temperature setpoint in the hypothalamus, not by actual overheating.
The cells most directly responsible are KNDy neurons in the arcuate nucleus of the hypothalamus, which express kisspeptin, neurokinin B, and dynorphin.
When estrogen declines, KNDy neurons hypertrophy and fire more often, releasing neurokinin B that activates NK3 receptors on nearby thermoregulatory neurons and triggers the heat-loss response we experience as a hot flash.
Fezolinetant, FDA-approved in 2023 based on the SKYLIGHT 1 and SKYLIGHT 2 Phase 3 trials, blocks the NK3 receptor and reduces hot flash frequency without using hormones.
About 75-80% of women experience hot flashes during the menopause transition, with substantial variation in frequency, intensity, and duration.
The short answer: what causes hot flashes
Hot flashes are caused by a narrowing of your hypothalamic thermoneutral zone — the range of core body temperature your brain treats as “normal.” When that zone narrows, small temperature changes that your brain would have ignored before now trigger an exaggerated heat-loss response: skin blood vessels dilate, you sweat, your heart rate climbs, and you feel a wave of heat. The trigger isn’t a real change in your environment. It’s that your brain’s thermostat has become hypersensitive.
What’s narrowing the zone is the loss of estrogen signaling to a specific cluster of neurons in the hypothalamus called KNDy neurons. Without estrogen’s restraining input, these neurons enlarge, fire more frequently, and release neurokinin B onto adjacent thermoregulatory neurons. That signal is what initiates the cascade we experience as a hot flash. This mechanism — confirmed across animal and human studies — is the rationale behind the new NK3 receptor antagonist class of non-hormonal treatments.
What a hot flash actually is
The clinical term for hot flashes and night sweats is vasomotor symptoms, or VMS. The name refers to the vascular smooth-muscle changes — vasodilation of skin blood vessels — that produce the sensation of heat. A typical hot flash lasts one to five minutes. It usually begins with a sudden sensation of warmth in the chest, neck, or face; flushing or visible redness of the skin; sweating, sometimes profuse; a transient rise in heart rate; and frequently a chill afterward as the body overcorrects and cools below baseline.
Most women experience their first hot flashes during late perimenopause, though they can begin in early perimenopause or even, in some cases, in the late reproductive stage. They can also appear for the first time after the final period. According to the Study of Women’s Health Across the Nation (SWAN), the longest-running cohort study of the menopause transition, vasomotor symptoms affect roughly three-quarters of women and last a median of 7.4 years. About 4.5 of those years occur after the final menstrual period. For roughly one in ten women, hot flashes persist into the 70s.
The intensity varies enormously. Some women have a handful of mild flashes a week. Others have ten or more daily, with drenching sweat that interrupts sleep, work, and conversation. Treatment decisions follow that gradient. For context on how long symptoms can last, our pillar on how long do hot flashes last breaks down the SWAN trajectory data.
The thermoneutral zone: what’s actually happening
Your hypothalamus, a small structure at the base of your brain, regulates core body temperature within a tight range. Below a certain temperature, you shiver. Above a certain temperature, you sweat. The zone between those two thresholds — where your body does neither — is called the thermoneutral zone.
In premenopausal women, this zone is wide enough that small fluctuations in core temperature don’t trigger either response. After the menopausal decline in estrogen, the zone narrows dramatically in women who experience hot flashes. Research by Robert Freedman and colleagues, published in Fertility and Sterility and subsequent work, established that women with frequent hot flashes have a much narrower thermoneutral zone than asymptomatic women. The same internal temperature change that would be inconsequential in one woman triggers an immediate heat-loss response — flushing, sweating, increased heart rate — in another.
This is why hot flash triggers are so individual. A small rise in skin temperature from a warm room, a hot drink, spicy food, a stress response, or even a small core-temperature change during the night can push you across the narrowed upper threshold and initiate a flash. For practical guidance on identifying your own pattern, our guide to hot flash triggers covers what’s worth tracking.
The thermoneutral zone framework also explains why the experience feels like overheating even though your actual core temperature is rarely elevated by much. Your brain has decided your temperature is too high; everything downstream — the sweating, flushing, heart rate change — is your body acting on that perception, not on objective reality.
The KNDy neuron discovery
The mechanism that narrows the thermoneutral zone has been worked out over the last twenty years, and the central player is a small population of neurons in the arcuate nucleus of the hypothalamus. These neurons co-express three signaling molecules: Kisspeptin, Neurokinin B, and Dynorphin. The acronym — KNDy, pronounced “candy” — was coined in the late 2000s after researchers including Naomi Rance and colleagues identified the cluster and characterized its function.
KNDy neurons are normally restrained by estrogen. Estrogen receptors on these cells receive ongoing input from circulating estradiol, which keeps the neurons relatively quiet and the cluster relatively small. When estrogen production declines during the menopause transition, that restraint is lost. Postmortem studies of postmenopausal women — including foundational work by Rance and colleagues published in Endocrinology and subsequent confirmation studies — show that KNDy neurons hypertrophy (literally grow larger) and increase their expression of neurokinin B.
The next step in the cascade involves a different population of neurons: the median preoptic nucleus (MnPO) neurons that drive heat-dissipation responses. KNDy neurons project to MnPO neurons and release neurokinin B onto them. These MnPO neurons express the neurokinin 3 receptor (NK3R) — the specific receptor that neurokinin B activates. When NK3R is activated, the MnPO neurons fire and initiate the cascade we experience as a hot flash: cutaneous vasodilation, sweating, increased heart rate.
This is the mechanism. The thermoneutral zone narrowing isn’t mysterious; it’s the downstream consequence of overactive KNDy neurons releasing too much neurokinin B onto NK3R-expressing thermoregulatory neurons. Estrogen replacement reverses the cascade by restoring estrogenic restraint on KNDy cells. NK3R antagonists like fezolinetant intervene further downstream, blocking the receptor itself.
The convergence of evidence is striking. Animal studies in which NK3R is pharmacologically blocked show reduced thermoregulatory responses, and Phase 2 and Phase 3 trials in humans have demonstrated that NK3R antagonism reduces hot flash frequency and severity, completing the translational loop.
Why estrogen withdrawal triggers all this
The brain is exquisitely sensitive to estrogen and progesterone. It's had these hormones in a particular pattern its whole life, and now that pattern is broken. That's not a character flaw. That's physiology.
It helps to step back and ask the obvious question: why does losing estrogen do this in the first place? The KNDy neuron system isn’t there to cause hot flashes. It’s there to regulate reproductive function. KNDy neurons are upstream regulators of GnRH (gonadotropin-releasing hormone) pulsatility, which drives the entire menstrual cycle. Estrogen’s feedback onto KNDy neurons is part of the normal cycle-control loop.
When ovarian estrogen production fails — gradually in natural menopause, abruptly in surgical menopause — that feedback is lost. The KNDy neurons “see” no estrogen and respond by amplifying their output. From a reproductive control standpoint, this is the body trying harder to drive GnRH and restore cyclicity. But because KNDy neurons also project to thermoregulatory circuitry, the amplified output spills over into temperature regulation. The thermoneutral zone narrows; hot flashes begin.
This is why the suddenness of estrogen loss matters. Women who undergo surgical menopause — bilateral oophorectomy — typically have more intense, more frequent, and earlier-onset hot flashes than women who go through the slower hormonal decline of natural menopause. The mechanism is the same; the magnitude of the trigger is larger. Our guide on surgical menopause covers the distinct clinical picture.
The same logic applies to medical menopause induced by chemotherapy, GnRH agonists, aromatase inhibitors, or tamoxifen. When these medications acutely lower estrogen activity, vasomotor symptoms can be severe — even more severe than typical menopausal hot flashes. For women on tamoxifen specifically, our overview of tamoxifen side effects covers symptom management options that are compatible with cancer treatment.
Why some women have hot flashes and others don’t
Roughly 75-80% of women experience some vasomotor symptoms during the menopause transition, but the range is wide: about 25% have severe, life-disrupting symptoms; about 50% have moderate symptoms; and about 25% have few or none. The factors that predict where you’ll fall have been studied extensively, mostly in SWAN.
Genetics. Polymorphisms in genes regulating KNDy signaling, estrogen metabolism, and serotonin transport have all been associated with VMS variation. Family history is one of the better predictors of your own symptom severity — if your mother and sisters had severe hot flashes, you’re more likely to as well.
Race and ethnicity. SWAN data show meaningful differences across groups: Black women have the longest median duration (10.1 years), Hispanic women 8.9, non-Hispanic white 6.5, Chinese 5.4, Japanese 4.8. The drivers — genetic, environmental, social, structural — remain under study.
Smoking. Current smoking is associated with earlier menopause, more frequent VMS, and longer duration. Quitting reduces VMS frequency, often within months.
Body mass index. Higher BMI is associated with both more frequent VMS and longer duration in SWAN. The dominant effect appears to involve thermoregulatory load. Weight loss can reduce hot flash frequency in women with elevated BMI.
Anxiety and depression. Both are associated with more severe VMS, though the direction of causation is complex: VMS disrupt sleep and mood, which amplify VMS distress, which worsens sleep and mood. Cognitive behavioral therapy targeting the appraisal of VMS rather than the flashes themselves has been shown to reduce symptom interference.
Surgical or medical induction. As above, abrupt estrogen withdrawal produces more intense VMS than gradual decline.
Stage of transition. Symptom intensity often peaks in late perimenopause and the first 1-2 years of postmenopause, then gradually attenuates for most women. The KNDy neuron system appears to slowly adapt over years to the new hormonal baseline.
What hot flash triggers actually do
Triggers don’t cause hot flashes in a strict mechanistic sense — the underlying cause is the narrowed thermoneutral zone — but they precipitate individual flashes by pushing core temperature, skin temperature, or stress response across the now-narrowed threshold. Common triggers include:
Ambient heat (warm rooms, hot showers, summer)
Hot drinks
The WHI unnecessarily frightened women and left them suffering with very real symptoms, possibly preventing them from taking a life-changing, even life-prolonging treatment. HRT can have beneficial impacts on bones, brains, and heart — and many common fears around hormone therapy are not supported by the latest scientific evidence.
Spicy food
Alcohol (especially red wine)
Caffeine
Emotional stress
Tight or non-breathable clothing
Going from a cold environment to a warm one
The triggers vary substantially between individuals, which is why personalized tracking is more useful than generic trigger lists. Identifying which triggers actually precipitate flashes for you allows targeted avoidance without unnecessary lifestyle restriction.
Treatment that works on the mechanism
Once you understand that hot flashes are caused by KNDy neuron overactivity acting on NK3 receptors in the thermoregulatory hypothalamus, treatment options sort into three categories.
Restore the upstream signal: estrogen therapy
Hormone therapy — replacing the estrogen that was driving the KNDy feedback loop — is the most effective treatment for VMS. According to The Menopause Society’s 2022 Hormone Therapy Position Statement, hormone therapy reduces hot flash frequency by approximately 75% in clinical trials and remains first-line for moderate-to-severe VMS in appropriately selected women under 60 or within ten years of their final period. It works because it does exactly what the loss of estrogen had undone: it quiets the KNDy neurons by restoring their inhibitory input.
NICE Guideline NG23 (updated November 2024) endorses the same position. Hormone therapy is not for everyone — contraindications include estrogen-sensitive cancers, a history of venous thromboembolism, active liver disease, and unexplained vaginal bleeding — but for women without contraindications, it’s the treatment with the largest effect size on the underlying mechanism. Our guides on HRT explained and HRT benefits walk through formulations, routes, and decision points in depth.
Block the downstream receptor: NK3 receptor antagonists
Fezolinetant, marketed as Veozah, is the first FDA-approved NK3R antagonist for moderate-to-severe vasomotor symptoms. Approval in 2023 followed the SKYLIGHT 1, 2, and 4 Phase 3 trials. SKYLIGHT 1, published by Lederman and colleagues in The Lancet in 2023, randomized 522 women to fezolinetant 30 mg, fezolinetant 45 mg, or placebo. Both doses produced significant reductions in hot flash frequency and severity by week 4 and sustained reductions through week 12. The 45 mg dose reduced daily moderate-to-severe hot flashes by approximately two-thirds compared with placebo. SKYLIGHT 2, published by Johnson and colleagues in the Journal of Clinical Endocrinology and Metabolism, produced essentially identical results in a separate cohort.
The mechanism is exactly what the KNDy model predicts: by blocking NK3R on MnPO thermoregulatory neurons, fezolinetant prevents the downstream signal that initiates hot flashes, without requiring any change in estrogen levels. This makes it useful for women who can’t or don’t want to use hormone therapy — including women with a history of breast cancer who can’t safely use systemic estrogen.
Fezolinetant has a few important practical considerations. It requires baseline and follow-up liver function testing because of rare elevations in liver enzymes observed in clinical trials. It interacts with strong CYP1A2 inhibitors (including fluvoxamine and certain antibiotics) and can’t be used concurrently with them. And like any prescription medication, it requires evaluation by a clinician who knows your medical history. For a full breakdown, see our guide to Veozah (fezolinetant).
Modulate temperature processing further upstream: SSRIs, SNRIs, gabapentin, oxybutynin
Several non-hormonal medications reduce VMS frequency through different mechanisms, none of which target KNDy neurons directly but all of which appear to modulate central temperature processing. Low-dose paroxetine (the only SSRI with FDA approval for VMS), escitalopram, venlafaxine, and desvenlafaxine all have evidence for moderate reductions in hot flash frequency. Gabapentin, originally an anticonvulsant, reduces nighttime VMS. Oxybutynin, an anticholinergic, reduces VMS through likely effects on sweat-gland innervation.
The Menopause Society’s 2023 Nonhormone Therapy Position Statement names these as evidence-supported options for women who can’t or don’t want hormones. The MsFLASH trial program, which conducted a series of randomized comparisons, provides much of the evidence base. Effect sizes are smaller than for hormone therapy or fezolinetant — typically 30-60% reductions in hot flash frequency — but meaningful for many women. Our guide on medication for hot flashes walks through the comparisons.
Behavioral and lifestyle approaches
Cognitive behavioral therapy targeting hot flashes (CBT-MS) has the strongest evidence among behavioral approaches. The MENOS 1 and MENOS 2 trials showed that CBT focused on changing the cognitive and emotional response to hot flashes reduces symptom interference and bother by clinically meaningful margins. CBT doesn’t reduce frequency much; it reduces how distressing they feel and how much they interfere with sleep and daily function.
Other approaches with some evidence include hypnosis, paced respiration during a flash, weight loss in women with elevated BMI, and reducing identified triggers. Our overview of natural remedies for menopause gives an honest evidence review of supplements and botanicals, most of which perform no better than placebo in rigorous trials.
What this means for you
Understanding the mechanism changes the clinical conversation in three useful ways.
First, it shifts the framing from “your hormones are imbalanced” to “specific neurons in your hypothalamus have lost an input that used to keep them quiet.” That’s not just semantic — it explains why the symptoms are so stereotyped, why certain treatments work, and why hot flashes are real neurophysiology, not a vague hormonal complaint.
Second, it explains why there’s now a non-hormonal medication that targets the actual cause. Until 2023, every non-hormonal option for VMS was repurposed from another indication: antidepressants, anticonvulsants, anticholinergics. Fezolinetant is the first treatment designed against the specific receptor that the mechanism named. The translational story — from postmortem KNDy neuron studies to a Phase 3 trial in The Lancet — took roughly two decades. It’s why the treatment landscape for VMS will continue to evolve.
Third, it helps you ask better questions of your clinician. If you’ve been told there’s “nothing to do” about hot flashes besides cutting caffeine and dressing in layers, that advice predates the current evidence. The realistic question is which treatment fits your situation — hormone therapy if you have no contraindications, fezolinetant if you can’t or won’t use hormones, evidence-supported non-hormonal medication if you need a third option, and behavioral approaches as either standalone or adjunctive treatment.
When to talk to your doctor
Reasonable triggers for a clinical conversation about hot flashes include:
Hot flashes that disrupt sleep more than two nights per week
Hot flashes that interfere with work, social function, or daily activity
New or worsening anxiety, depression, or cognitive symptoms accompanying VMS
VMS that started before age 45, particularly before age 40
VMS following surgical removal of the ovaries, chemotherapy, or hormone-blocking treatment
VMS accompanied by unusual symptoms — chest pain, palpitations, new headaches, weight loss — that warrant ruling out other causes
Wanting to discuss whether to start, change, or stop hormone therapy
If you’re preparing for that conversation, our guide on talking to your doctor about menopause covers what to bring, what to ask, and how to advocate for treatment if you’ve been dismissed.
Frequently asked questions
What causes hot flashes in menopause?
The proximate cause is overactivity of KNDy neurons in the hypothalamus, which release neurokinin B onto NK3 receptors on thermoregulatory neurons. The underlying trigger is the loss of estrogen’s restraining input on those KNDy neurons. The downstream consequence is a narrowed thermoneutral zone, which means small temperature changes trigger an exaggerated heat-loss response — the hot flash.
Why do I get hot flashes at night?
Nighttime hot flashes (night sweats) work through the same mechanism as daytime ones but are made more noticeable by sleep position, bedding, and the small core-temperature rises that occur during certain sleep stages. They’re also more disruptive because they interrupt sleep. The biology is the same; the context amplifies the impact.
Can men get hot flashes?
Yes, in specific clinical contexts. Men receiving androgen deprivation therapy for prostate cancer often experience hot flashes through an analogous mechanism — sudden loss of sex hormone input on hypothalamic thermoregulation. The treatment approaches overlap with those used in menopausal women, though prescribing decisions differ.
Do hot flashes mean my estrogen is low?
In one sense yes, in another sense the question misses the point. Your estrogen is lower than it was, but the more specific answer is that your KNDy neurons have lost the estrogenic input they were calibrated to expect, and they’re now firing more often. The hot flash isn’t directly caused by the absolute estrogen level; it’s caused by the downstream neuronal response to that change.
Will hot flashes ever go away on their own?
For most women, yes. SWAN data show a median total VMS duration of 7.4 years, with substantial variation. About one in ten women continue to experience hot flashes into their 70s. Whether to wait it out or treat depends on how much they’re affecting your life — the median timeline isn’t short.
Is fezolinetant safer than hormone therapy?
“Safer” is the wrong frame — both are reasonable treatments with different risk profiles. Fezolinetant doesn’t carry the cancer or thrombosis considerations of estrogen but does require liver function monitoring and has its own contraindications and drug interactions. Hormone therapy has more long-term data and additional benefits beyond VMS (bone, GSM). Choice depends on your medical history, contraindications, and preferences. For more, see our guide on is HRT safe.
Sources
Avis NE, Crawford SL, Greendale G, et al. Duration of menopausal vasomotor symptoms over the menopause transition. JAMA Internal Medicine. 2015;175(4):531-539. SWAN cohort.
Freedman RR, Krell W. Reduced thermoregulatory null zone in postmenopausal women with hot flashes. American Journal of Obstetrics and Gynecology. 1999;181(1):66-70.
Rance NE, Young WS. Hypertrophy and increased gene expression of neurons containing neurokinin-B and substance-P messenger ribonucleic acids in the hypothalami of postmenopausal women. Endocrinology. 1991;128(5):2239-2247.
Lederman S, Ottery FD, Cano A, et al. Fezolinetant for treatment of moderate-to-severe vasomotor symptoms associated with menopause (SKYLIGHT 1): a phase 3 randomised controlled study. The Lancet. 2023;401(10382):1091-1102.
Johnson KA, Martin N, Nappi RE, et al. Efficacy and safety of fezolinetant in moderate to severe vasomotor symptoms associated with menopause: a phase 3 RCT (SKYLIGHT 2). Journal of Clinical Endocrinology and Metabolism. 2023;108(8):1981-1997.
The Menopause Society. The 2022 Hormone Therapy Position Statement of The North American Menopause Society. Menopause. 2022;29(7):767-794.
The Menopause Society. The 2023 Nonhormone Therapy Position Statement of The North American Menopause Society. Menopause. 2023;30(6):573-590.
Joffe H, Guthrie KA, LaCroix AZ, et al. Low-dose estradiol and the serotonin-norepinephrine reuptake inhibitor venlafaxine for vasomotor symptoms (MsFLASH). JAMA Internal Medicine. 2014;174(7):1058-1066.
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National Institute for Health and Care Excellence. Menopause: identification and management (NG23). Updated November 2024.
Disclaimer
This article is for educational purposes and does not constitute medical advice. Always consult a qualified healthcare provider for guidance on your specific situation. Prescription medications referenced in this article — including hormone therapy, fezolinetant, SSRIs, SNRIs, gabapentin, and oxybutynin — require evaluation by a clinician, have specific contraindications and monitoring requirements, and are not appropriate for everyone. Fezolinetant specifically requires baseline and follow-up liver function testing and has clinically significant drug interactions.
