Sex hormones are a small family of chemical messengers, built from cholesterol, that orchestrate reproduction, shape the body’s structure, and influence nearly every organ system from the brain to the bones. The three main groups are estrogens, androgens, and progestogens. Although people tend to think of estrogen as “the female hormone” and testosterone as “the male hormone,” every human body produces all three types, just in different proportions. What makes these molecules so powerful is not just their role in fertility but their reach: they affect how your heart functions, how dense your bones are, how your immune system responds to infection, and even how your brain is wired.
Where Sex Hormones Come From
All sex hormones are steroids, meaning they share a core structure derived from cholesterol. The main production sites are the ovaries, the testes, and the adrenal glands, though smaller amounts are made locally in tissues like fat and brain. In response to signals from the brain, these glands convert cholesterol through a chain of enzymatic steps into the specific hormones the body needs at a given moment.1PubMed Central. Cellular cholesterol delivery, intracellular processing and utilization for biosynthesis of steroid hormones The process is tightly regulated by a feedback loop between the brain and the gonads, known as the hypothalamic-pituitary-gonadal axis. Specialized neurons in the hypothalamus release signaling peptides that tell the pituitary gland to secrete hormones called LH and FSH, which in turn tell the ovaries or testes to produce sex steroids. Those steroids then feed back to the brain to dial their own production up or down.2PubMed Central. Emerging insights into hypothalamic-pituitary-gonadal axis regulation and interaction with stress signalling
This feedback loop explains why the system is so sensitive to disruption. Chronic stress, extreme weight loss, sleep deprivation, and certain medications can all suppress the brain’s signals and lower sex hormone output, sometimes dramatically. It also explains how hormonal contraception works: supplying external hormones tells the brain that enough is already circulating, which shuts down the signals that would otherwise trigger ovulation.
The Main Players and How They Work
Estrogens, androgens, and progestogens each have distinct receptors scattered across the body, and each triggers different cellular responses depending on which tissue it lands in.
Estrogens, the most prominent being estradiol, were long understood to work by entering a cell’s nucleus and switching genes on or off. That is still true, but researchers have discovered that estrogens also trigger rapid signaling events at the cell surface through a membrane receptor called GPER, producing effects within seconds or minutes rather than hours.3PubMed Central. Estrogen biology: new insights into GPER function and clinical opportunities This dual action, one slow and gene-based, one fast and membrane-based, helps explain how estrogen can simultaneously influence long-term processes like bone remodeling and immediate events like blood vessel dilation.4PubMed Central. Estrogen receptor signaling mechanisms
Androgens work through the androgen receptor. Testosterone is the most well-known, but in many tissues it is converted into dihydrotestosterone (DHT), which binds the androgen receptor more tightly and produces a stronger effect. DHT is the hormone primarily responsible for male reproductive tract development and traits like facial hair growth and prostate function.5PubMed Central. Modulation of androgen receptor activation function 2 by testosterone and dihydrotestosterone Many of testosterone’s effects elsewhere in the body, including on muscle, bone, and even mood, are partly mediated through its conversion to either DHT or estradiol.6PubMed Central. Relation of Testosterone, Dihydrotestosterone, and Estradiol With Changes in Outcomes Measures in the Testosterone Trials That last point surprises people: testosterone is actually a precursor for estrogen. An enzyme called aromatase converts testosterone into estradiol in tissues like fat, bone, and brain, which means that men’s estrogen levels partly depend on their testosterone levels.
Progesterone plays its most recognized role in pregnancy, where it primes the uterine lining for embryo implantation and maintains early gestation. But progesterone signaling also appears to protect against uterine diseases like endometrial cancer and endometriosis when functioning properly.7PubMed Central. Progesterone receptor signaling in the initiation of pregnancy and preservation of a healthy uterus Beyond reproduction, progesterone has calming effects on the brain, partly because it is converted into a neurosteroid called allopregnanolone that modulates a major inhibitory brain receptor.
The Menstrual Cycle as a Hormonal Symphony
The menstrual cycle is the clearest real-time illustration of how sex hormones rise, fall, and interact. During the first half of the cycle, estrogen climbs as a follicle matures in the ovary, while progesterone stays low. Around mid-cycle, a surge of LH and FSH from the pituitary triggers ovulation, and estrogen briefly dips. After ovulation, the leftover follicle transforms into a structure that pumps out both progesterone and estrogen during the second half of the cycle. If no pregnancy occurs, both hormones plummet, the uterine lining sheds, and the cycle resets.8The American Journal of Clinical Nutrition. Physiology of the menstrual cycle
These fluctuations do not just affect the uterus. Recent imaging research shows that estradiol and progesterone levels influence the activity of several large-scale brain networks throughout the cycle, including those involved in attention, body awareness, and emotional processing.9npj Women’s Health. Whole-brain dynamics across the menstrual cycle: the role of hormonal fluctuations and age in healthy women This helps explain why some women notice shifts in focus, mood, or energy at predictable points in their cycle, though the magnitude of these shifts varies enormously between individuals.
Beyond Reproduction
One of the most important things to understand about sex hormones is how much they do outside the reproductive system. Their receptors are found in bone, heart, liver, fat, immune cells, and brain tissue, which means their influence extends across the entire body.
Bone
Estrogen is critical for maintaining bone density in both men and women. It works in part by suppressing a signaling molecule called RANKL that activates cells responsible for breaking down bone.10PubMed Central. Estrogen Regulates Bone Turnover by Targeting RANKL Expression in Bone Lining Cells When estrogen drops, that brake is released, and bone breakdown accelerates.11PubMed. Estrogens suppress RANK ligand-induced osteoclast differentiation via a stromal cell independent mechanism involving c-Jun repression This is why postmenopausal women face a sharply increased risk of osteoporosis, and why men with unusually low estrogen (not just low testosterone) also lose bone. Testosterone contributes to bone health as well, both directly and through its conversion to estradiol.
Heart and Blood Vessels
Before menopause, women have substantially lower rates of cardiovascular disease than men of the same age. After menopause, that gap narrows and eventually reverses, with postmenopausal women developing cardiovascular disease at higher rates than men. This pattern has been attributed to estrogen’s protective effects on blood vessels, including its role in reducing oxidative stress and promoting healthy vessel function.12PubMed Central. Protective Effects of Estrogen on Cardiovascular Disease Mediated by Oxidative Stress
Immune Function
Sex hormones contribute to a well-documented split in immune behavior between men and women. Women mount stronger immune responses to both infections and vaccines, but they also develop autoimmune diseases at far higher rates. Both innate and adaptive immune cells carry receptors for sex hormones and respond to hormonal signals, which helps explain why the immune landscape shifts during pregnancy, puberty, and menopause.13PubMed Central. Sex Hormones in Acquired Immunity and Autoimmune Disease This is not simply a matter of estrogen boosting immunity. The relationship is dose- and context-dependent: high estrogen levels during pregnancy, for instance, tend to suppress certain immune pathways to prevent rejection of the fetus.
Metabolism and Body Composition
Testosterone and estrogen both influence where and how much fat the body stores. In men, lower testosterone is associated with increased abdominal fat and reduced glucose disposal, linking low androgens to metabolic risk.14PubMed. Insulin resistance, body fat distribution, and sex hormones in men However, whether low testosterone directly causes insulin resistance or is mainly a marker of excess body fat remains debated. One study using detailed body composition measurements found that once you accounted for how much fat a man carried, the link between testosterone levels and insulin resistance largely disappeared.15The Journal of Clinical Endocrinology & Metabolism. Sex Steroid Hormones, Upper Body Obesity, and Insulin Resistance This matters practically because it suggests that losing weight may do more to improve metabolic health than supplementing testosterone alone.
How Sex Hormones Shape the Brain
During early development, sex hormones leave a permanent imprint on brain structure and function. Gonadal steroids act on the developing brain to create lasting differences that later dictate how the brain responds to hormones in adulthood.16PubMed Central. How it’s made: organisational effects of hormones on the developing brain In rodents, and likely in humans to varying degrees, estradiol is the primary hormone sculpting the male hypothalamus during a critical period around birth, shaping differences in cell number, connectivity, and neurotransmitter expression.17PubMed Central. Organized for sex – steroid hormones and the developing hypothalamus The fact that estradiol, rather than testosterone itself, is the key masculinizing signal in the brain is one of the more counterintuitive findings in neuroendocrinology. Testosterone produced by the fetal testes is converted to estradiol inside brain cells, and it is this locally produced estradiol that drives the process.
In adulthood, fluctuating sex hormones continue to affect mood, cognition, and mental health. Women face roughly twice the risk of depression and anxiety disorders compared to men, and the periods of greatest vulnerability, puberty, the premenstrual phase, the postpartum period, and perimenopause, all coincide with times of dramatic hormonal change.18PubMed Central. Sex hormone fluctuation and increased female risk for depression and anxiety disorders: From clinical evidence to molecular mechanisms The mechanisms are still being untangled, but sex hormones influence neurotransmitter systems involved in mood regulation, including serotonin and dopamine pathways.19PubMed Central. Effects of Estrogens on Central Nervous System Neurotransmission: Implications for Sex Differences in Mental Disorders
What Happens When Hormones Decline With Age
In women, the decline is steep. Estrogen production drops markedly during perimenopause and falls to very low levels after the final menstrual period. This loss has been linked to a higher risk of metabolic disease, bone loss, and increased mortality, beyond what aging alone would predict.20PubMed Central. Exploring the effects of estrogen deficiency and aging on organismal homeostasis during menopause Hot flashes, sleep disruption, vaginal dryness, and mood changes are among the most common symptoms, and they vary widely in severity from one person to the next.
In men, the decline is more gradual and less universal. Testosterone dips slowly after about age 30, but only a small proportion of aging men develop a genuine clinical syndrome of low testosterone with sexual, physical, and psychological symptoms.21PubMed Central. Late-onset hypogonadism: current concepts and controversies of pathogenesis, diagnosis and treatment A large European study found that the symptoms most reliably tied to declining testosterone were sexual, specifically poor morning erections, low desire, and erectile dysfunction, and these only became clearly more common below specific testosterone thresholds.22PubMed. Identification of late-onset hypogonadism in middle-aged and elderly men Importantly, obesity and chronic disease are often bigger drivers of low testosterone in older men than age itself.23PubMed Central. Late-onset hypogonadism: current concepts and controversies of pathogenesis, diagnosis and treatment A 65-year-old man in good health with a healthy weight may have perfectly normal testosterone levels, while a sedentary 50-year-old with obesity and type 2 diabetes might test well below the normal range.
Hormone Therapy After Menopause
Menopausal hormone therapy has been one of the most debated areas of medicine over the past two decades. The current understanding is more nuanced than the panic that followed early large trials. Hormone therapy effectively treats hot flashes, vaginal symptoms, and prevents osteoporotic fractures. For healthy women who start treatment within about ten years of menopause, the benefits generally outweigh the risks, including potential cardiovascular protection and improved bone density.24Human Reproduction Update. Hormone therapy in the postmenopausal years: considering benefits and risks in clinical practice
Risk depends heavily on the specifics. Transdermal estradiol, delivered through a skin patch, carries a lower blood-clot risk than oral formulations. Natural progesterone appears safer for breast tissue than some synthetic progestins. Estrogen-only therapy, used in women who have had a hysterectomy, has a more favorable breast cancer profile than combined estrogen-progestin regimens, where risk increases with longer use.25PubMed Central. Menopausal Hormone Therapy-Risks, Benefits and Emerging Options: A Narrative Review Women with premature menopause, those who lose ovarian function before age 40, stand to gain the most from replacement, as the early and prolonged absence of estrogen accelerates bone and cardiovascular aging significantly.
Sex Hormones in Gender-Affirming Care
Gender-affirming hormone therapy provides a real-world window into what sex hormones do to the body when the dominant hormonal profile is deliberately shifted. In transgender men receiving testosterone, lean body mass increases, fat mass drops, and visceral (abdominal) fat tends to rise, along with changes in lipid profiles, including higher LDL cholesterol and lower HDL cholesterol over several years.26PubMed Central. Longitudinal changes in regional fat and muscle composition and cardiometabolic biomarkers over 5 years of hormone therapy in transgender individuals In transgender women receiving estrogen and anti-androgens, overall fat increases but visceral fat decreases, lean mass drops, and fat redistribution moves toward a typically female pattern. Skeletal muscle size increased by about a fifth in transgender men after six years while declining modestly in transgender women, and muscle strength followed a similar trajectory.27PubMed Central. Longitudinal changes in regional fat and muscle composition and cardiometabolic biomarkers over 5 years of hormone therapy in transgender individuals
These changes underscore how powerfully sex hormones shape body composition independently of genetics, diet, or exercise. They also highlight trade-offs: testosterone therapy, for instance, improved body composition and strength but came with shifts in cardiovascular risk markers that need monitoring.28PubMed Central. Nutritional Monitoring During Gender-Affirming Hormone Therapy: Body Composition and Metabolic Implications
Estrogen, Muscle, and Injury Risk
Estrogen’s effects on the musculoskeletal system are a study in trade-offs. It supports muscle mass and strength, and evidence from both human and animal research points to its role in protecting muscle fibers from damage during exercise and aiding recovery afterward.29PubMed Central. Benefits of estrogen replacement for skeletal muscle mass and function in post-menopausal females: evidence from human and animal studies Estrogen also promotes satellite cell activity, which is crucial for muscle repair after injury.30PubMed. How sex hormones promote skeletal muscle regeneration
But estrogen also reduces the stiffness of tendons and ligaments by altering collagen cross-linking. While more flexible connective tissue is not inherently bad, lower stiffness can reduce power output and make joints less stable under high loads. This is one reason women have significantly higher rates of anterior cruciate ligament tears, and the risk appears to fluctuate across the menstrual cycle as estrogen levels change.31PubMed Central. Effect of Estrogen on Musculoskeletal Performance and Injury Risk The practical implication for athletes and coaches is that training programs may benefit from accounting for cycle phase, particularly around periods of peak estrogen, though research on how best to implement this is still evolving.
Endocrine Disruptors and Environmental Exposure
A wide range of natural and synthetic chemicals in the environment can mimic or block sex hormones. These endocrine-disrupting chemicals bind to estrogen or androgen receptors, and some can also interfere with the proteins that transport sex hormones through the bloodstream.32PubMed Central. Environmental xenobiotics may disrupt normal endocrine function by interfering with the binding of physiological ligands to steroid receptors and binding proteins Compounds like certain pesticide residues, industrial chemicals, and plasticizers have been shown to activate estrogen receptors or block androgen receptors in laboratory settings.33PubMed Central. Assessment and molecular actions of endocrine-disrupting chemicals that interfere with estrogen receptor pathways
Translating these laboratory findings into real-world health effects has proven difficult, however. Despite decades of concern about falling sperm counts and rising reproductive abnormalities, analyses of long-term data have shown significant geographic variation in sperm counts rather than a universal decline, and rates of certain reproductive tract abnormalities have been stable or even declining since the mid-1980s in studied populations.34PubMed Central. Endocrine disruptors and human health–is there a problem? An update. The honest assessment is that while individual chemicals clearly can interfere with hormone signaling in controlled experiments, proving that real-world exposures at typical environmental levels cause population-wide reproductive harm has been much harder than early warnings suggested. The field remains active and contested.
An Ancient Signaling System
Sex hormone receptors are not a human invention. Genome analyses across the animal kingdom show that an estrogen receptor first appeared in basal chordates, the simple ancestors of all vertebrates. A separate ancestor receptor later duplicated and gave rise to the progesterone receptor, then the androgen receptor, and eventually distinct receptors for stress hormones as well. The full complement of sex steroid receptors we recognize today, including separate estrogen receptor subtypes, was already in place by the time cartilaginous fish like sharks evolved.35PubMed. Steroid receptors and vertebrate evolution This deep evolutionary history explains why sex hormones regulate so many different tissues: the receptor system has been layered into new biological roles for hundreds of millions of years. It also explains why endocrine disruptors affect such a broad range of species. Fish, amphibians, reptiles, and mammals all share fundamentally similar receptor machinery, which means a chemical that mimics estrogen in a human cell can also do so in a fish or a frog.