Hair type can and does change over a person’s lifetime, sometimes dramatically. Hormones, aging, medications, medical conditions, and even scalp inflammation can all alter whether your hair grows in straight, wavy, or curly. These shifts are not just cosmetic illusions from damage or styling habits; they often reflect real changes at the level of the hair follicle itself. The science behind why this happens turns out to be more complex than most people expect.
What Actually Determines Whether Hair Is Straight or Curly
The shape of your hair follicle is the primary driver of your curl pattern. A round follicle tends to produce straight hair, while an oval or asymmetric follicle produces waves or curls. But follicle shape alone doesn’t tell the whole story. Research into the biomechanics of the mature hair shaft shows that the degree to which different cell types segregate along the shaft, and how the plane of that segregation shifts dynamically, can account for complex curl patterns. An irregular, flattened cross-section plays a role but isn’t the deciding factor on its own.1PubMed. Why is hair curly?-Deductions from the structure and the biomechanics of the mature hair shaft
This matters because anything that alters follicle geometry or the internal architecture of the growing hair fiber can change how your hair looks and behaves when it emerges. It also explains why hair texture can shift gradually rather than all at once: the follicle doesn’t flip a switch. It remodels over time in response to signals from hormones, inflammation, or aging.
Puberty and the First Big Shift
Many people first notice a change in hair type during puberty. A child with pin-straight hair might develop waves or curls as a teenager, or vice versa. The mechanism is androgen-driven. During puberty, rising levels of androgens (like testosterone and its derivatives) harness the hair follicle’s regenerative cycle to replace many small, fine vellus hairs with larger, darker terminal hairs.2PubMed Central. Androgens trigger different growth responses in genetically identical human hair follicles in organ culture that reflect their epigenetic diversity in life This is most obvious with the appearance of body and facial hair, but the same hormonal surge affects scalp follicles too. As follicles enlarge and reshape in response to androgens, the cross-sectional geometry of the hair they produce can shift, changing curl pattern, thickness, or both.
What’s interesting is that genetically identical follicles on the same person can respond to androgens in completely different ways, depending on their epigenetic programming. That is part of why the change doesn’t happen uniformly across your head and why some people get a dramatic shift while others barely notice one.
Pregnancy and Postpartum Hair
Pregnancy is another hormonal event that commonly reshapes how hair looks and feels. During pregnancy, elevated estrogen and other hormones keep an increased number of hair follicles in their active growth phase for longer than usual. After delivery, those follicles shift back to the resting phase within about three to six months as hormone levels drop.3PubMed. The changes in the hair cycle during gestation and the post-partum period The classic result is the “thicker, fuller hair” many pregnant women notice, followed by what feels like alarming hair loss postpartum. But beyond volume, the texture itself can change. Some women report their hair going from straight to wavy, or from curly to straighter, during pregnancy. These changes can be temporary, resolving within a year or so after delivery, or they can persist.
The persistence question is one researchers haven’t fully answered. If the follicle was merely delayed in its cycle, you’d expect the original texture to return once the cycle normalizes. But if the hormonal environment during pregnancy physically remodeled the follicle, the new texture may stick around. Anecdotally, many women say their hair “never went back” after pregnancy, and the mechanism for that lasting change isn’t well understood.
Menopause and the Gradual Thinning Effect
Menopause represents a longer and more gradual hormonal transition, and its effects on hair tend to be cumulative. As estrogen levels decline, follicles can miniaturize, growth rates slow, and individual hairs may become finer in diameter.4PubMed Central. Hair Aging in Different Races and Ethnicities The result is often thinning, loss of volume, and changes in texture, with many women reporting their hair becomes drier, coarser in feel (even as it gets finer), or less defined in its wave pattern.5PubMed Central. The Menopausal Transition: Is the Hair Follicle “Going through Menopause”?
These changes overlap with the general effects of aging on hair, which makes it difficult to tease apart what is hormonal and what is simply time-related. The practical reality is that both contribute simultaneously, and the combined effect is why hair at sixty rarely looks or feels like hair at thirty, even without any chemical treatment.
What Aging Does to the Follicle Itself
Beyond hormonal shifts, aging attacks the hair follicle at a structural level. Research in both mice and humans has shown that hair follicle stem cells, which are responsible for regenerating the follicle through each growth cycle, gradually lose their stemness over time. DNA damage triggers the breakdown of a key structural protein called type XVII collagen, which is critical for maintaining those stem cells. As the protein degrades, aged stem cells are pushed out of the follicle through a process that essentially converts them into surface skin cells. The result is stepwise follicle miniaturization and, eventually, hair loss.6PubMed. Hair follicle aging is driven by transepidermal elimination of stem cells via COL17A1 proteolysis
This miniaturization doesn’t just thin hair; it can change its texture. A follicle that once produced a thick, well-defined curl might now produce a fine, wispy fiber that behaves entirely differently. Graying compounds this perception, since melanin-free hair has different surface properties and often feels coarser or wiry, even when the follicle itself hasn’t changed its shape.
Chemotherapy and Hair That Grows Back Different
One of the most striking real-world examples of hair type change happens after chemotherapy. Patients who lose their hair during treatment commonly report that the hair growing back has a completely different texture, sometimes called “chemo curls.” Straight hair may come back curly, or curly hair may return straighter. The mechanism involves chemotherapy’s effect on the rapidly dividing cells in the hair matrix. The degree of damage to hair follicle stem cells determines whether the resulting hair loss is reversible, and chemotherapy-induced hair-cycle abnormalities driven by different damage pathways shape the hair regrowth pattern.7The Lancet Oncology. Chemotherapy-induced alopecia
For many patients, the texture change is temporary, with hair gradually returning to something closer to its pre-treatment state over one to two years. But for some, the new texture persists indefinitely. The working theory is that chemotherapy damages follicle stem cells unevenly, altering the geometry and cell-type composition of the regenerating follicle. Those structural changes then dictate the curl pattern of whatever hair grows back.
Other Medications That Alter Texture
Chemotherapy isn’t the only class of drugs that changes hair texture. A systematic review of the literature found that hair texture changes were associated with several medication categories. Cancer drugs (antineoplastics) accounted for the most reported cases, but anti-epileptic drugs, retinoids, and immunomodulators also showed up. Across these reports, the most common change was the development of new or exaggerated curling patterns.8PubMed. Systemic Medications Associated With Hair Texture Changes Individual case reports continue to expand this list. For example, pomalidomide, a drug used for multiple myeloma, was recently linked to a straight-to-curly shift in a patient whose hair texture had never changed before.9PubMed. Change in hair texture with the use of pomalidomide for multiple myeloma: From straight to curly phenotype
Why so many different drugs produce similar curling effects remains an open question. One hypothesis is that any systemic agent that disrupts the cell division patterns in the hair matrix can alter the internal asymmetry of the fiber, nudging it toward a curled configuration. But the evidence is thin enough that researchers frame this as an area needing more investigation rather than a settled mechanism.
Thyroid Disease and Endocrine Disorders
Your thyroid gland affects almost every tissue in your body, and hair is no exception. Hypothyroidism, in which the thyroid produces too little hormone, commonly leads to hair that becomes dry, brittle, and coarser in texture.10PubMed Central. Dermatologic manifestations of endocrine disorders Hyperthyroidism can cause hair to become finer and more fragile. In both cases, the texture change is often one of the earlier noticeable symptoms, appearing before a person even suspects they have a thyroid problem. The good news is that these changes tend to reverse once thyroid hormone levels are brought back into a normal range with treatment, though recovery can take months because hair grows slowly.
Other endocrine conditions, including polycystic ovary syndrome (PCOS) and adrenal disorders that alter androgen levels, can similarly shift hair texture or thickness. The common thread is that the follicle is exquisitely sensitive to circulating hormones, and when those hormones are out of balance, the follicle’s output changes accordingly.
Scalp Conditions and Inflammation
You might not think of dandruff or psoriasis as conditions that change your hair type, but chronic scalp inflammation can measurably alter the hair shaft. In seborrheic dermatitis, atomic force microscopy studies have shown that affected hairs have significantly damaged cuticles, with scale thickness seven times greater than in healthy control hairs, and hair diameter reduced by roughly 10 to 35 percent compared to nearby unaffected hair.11PubMed. Investigation of hair shaft in seborrheic dermatitis using atomic force microscopy Scalp psoriasis produces even more dramatic surface damage, with all affected hairs showing macropits on the shaft and scale thickness more than four-fold that of normal hair.12PubMed. A comparative study of hair shafts in scalp psoriasis and seborrheic dermatitis using atomic force microscopy
This kind of damage doesn’t just make hair look unhealthy. It changes how hair behaves. Damaged cuticles alter the fiber’s friction, flexibility, and moisture retention, which collectively affect how the hair moves and clumps. Someone with a chronic scalp condition might notice their hair feels rougher, tangles more easily, or holds a wave differently than it used to. Treating the underlying condition often restores normal hair-shaft quality over several growth cycles.
Chemical Treatments and Structural Damage
Hair dyes, perms, and chemical relaxers alter hair’s shape and appearance by mechanically changing the physical structure and chemical makeup of the shaft. While these cosmetic procedures are generally safe, they can cause structural damage, disrupt the hair’s chemical constituents, and impair its physical properties.13PubMed Central. Mechanisms of impairment in hair and scalp induced by hair dyeing and perming and potential interventions It’s worth understanding the distinction between these chemical treatments and the biological changes discussed above. A perm or relaxer changes the shape of hair that has already grown out of the follicle. It does not change the follicle itself. New growth will revert to your natural texture, which is why these treatments need regular touch-ups.
Heat styling operates similarly. A flat iron or curling iron temporarily reshapes the hydrogen bonds in the hair shaft. With repeated use at high temperatures, cumulative damage can alter how the hair feels and behaves between styling sessions, giving the impression that the natural texture has changed. But the follicle is still producing the same fiber; it’s the shaft that’s been modified. This is a fundamentally different situation from, say, a hormonal shift that actually remodels the follicle.
Sun Exposure and Environmental Weathering
Prolonged sun exposure degrades hair in ways that can subtly change its texture. Solar radiation affects color, mechanical properties, luster, protein content, and surface roughness, though there is no consensus yet on which wavelengths of sunlight cause which specific types of damage.14PubMed Central. Effects of solar radiation on hair and photoprotection The practical effect is familiar to anyone who spends a lot of time outdoors: hair becomes drier, more porous, lighter in color, and more prone to breakage. As the outer cuticle layer erodes, hair may feel rougher and respond differently to humidity and styling. Like chemical damage, this is a shaft-level effect rather than a follicle-level one, but it contributes to the perception that your hair type has “changed.”
Ethnic Variation in Hair Chemistry
Hair texture varies between ethnic groups not just in obvious ways like curl diameter but also at the chemical level. Thermoanalytical studies comparing hair from different ethnic backgrounds have found, for instance, that hair from people of European descent tends to have higher water content in the fiber, while hair from people of African descent shows signs of potentially lower cysteine disulfide bond content in the hair matrix.15PubMed. Ethnic hair: Thermoanalytical and spectroscopic differences These chemical differences help explain why different hair types respond differently to humidity, heat, and chemical treatments. They also mean that the same triggering event, whether it’s a hormonal change, a medication, or sun damage, may produce different texture changes depending on your baseline hair chemistry.
Researchers have been working on more objective ways to classify and measure hair texture. Micro-CT scanning, for example, can now resolve the cross-sectional shape of individual hair fibers in detail, revealing that straighter hair types actually show greater variability along a single fiber than tightly curled hair does.16PubMed Central. Quantifying whole human hair scalp fibres of varying curl: A micro-computed tomographic study Better measurement tools are gradually replacing the subjective classification systems that have historically been used, which should help researchers track texture changes more precisely in clinical studies.
Uncombable Hair Syndrome and Rare Genetic Conditions
At the far end of the spectrum are rare genetic conditions that produce highly unusual hair textures from birth. Uncombable hair syndrome (UHS) is the best known, producing hair that grows out from the scalp in all directions and resists being flattened, with a distinctive silvery-blond, straw-like appearance. Genetic studies have traced UHS to mutations in three genes involved in hair-shaft formation: PADI3, TGM3, and TCHH. The condition follows an autosomal-recessive inheritance pattern in the majority of cases, meaning both parents must carry a copy of the relevant mutation.17American Journal of Human Genetics. Mutations in Three Genes Encoding Proteins Involved in Hair Shaft Formation Cause Uncombable Hair Syndrome
A larger genetic study of over 100 individuals with UHS expanded the known mutation spectrum considerably, identifying previously unreported variants in the PADI3 gene alone across individuals from 20 countries.18JAMA Dermatology. Assessment of the Genetic Spectrum of Uncombable Hair Syndrome in a Cohort of 107 Individuals The condition usually improves on its own by adolescence, which adds an interesting wrinkle to the question of whether hair type can change. For children with UHS, the change is dramatic and appears to be driven partly by developmental maturation of the hair follicle rather than any treatment.
Why Tightly Curled Hair May Have Been an Evolutionary Advantage
The diversity of human hair types isn’t just a cosmetic accident. Recent experimental work using a thermal manikin fitted with human hair wigs suggests that scalp hair functions as a thermoregulatory adaptation, reducing solar heat gain while minimally impeding sweat evaporation. Tightly curled hair performed especially well in this role, offering the greatest protection against incoming solar radiation compared to straighter textures.19PubMed Central. Human scalp hair as a thermoregulatory adaptation While all hair types reduced heat gain, the curled morphology created a thicker insulating layer above the scalp with less total fiber mass, shading the skin while allowing air to circulate.
This finding supports the idea that variation in human hair texture was shaped by climate over long evolutionary timescales, with populations in equatorial regions developing tighter curls as protection against intense overhead sun. It also puts modern hair-type changes in perspective. The follicle’s ability to shift texture in response to hormones, aging, or other signals may reflect the same underlying plasticity that allowed human hair to diversify across environments in the first place. The follicle, in other words, was never designed to produce one fixed output for life. It was built to respond.