Hair that was once pin-straight can genuinely become wavy or curly over time, and the shift is not your imagination. The change originates in the hair follicle itself, where hormonal fluctuations, aging, genetic expression, and even certain medications can alter the shape of the tiny tunnel from which each strand grows. A rounder follicle produces straighter hair; a more oval or asymmetric follicle produces curlier hair. When something reshapes that follicle or changes the internal structure of the growing fiber, the hair that emerges can look and feel completely different from what you had a few years ago.
What Actually Determines Whether Hair Is Straight or Curly
The curl of a hair strand is set before it ever leaves your scalp. Inside the follicle, the growing fiber is made up of two types of cells arranged along its length. Research on wool fibers, which share the same basic keratin biology as human hair, has shown that curvature depends on the difference in length between these two cell types: the cells on the outer edge of a curve are longer than those on the inner edge, creating a built-in bend much like how a bimetallic strip curls when heated unevenly.1PubMed. Intrinsic curvature in wool fibres is determined by the relative length of orthocortical and paracortical cells The greater the mismatch, the tighter the curl.
The shape of the follicle canal matters too. Follicle morphology varies across populations: studies measuring follicle dimensions in different ethnic groups have found that terminal hair follicles differ in size, and the cross-sectional shape of the follicle canal ranges from nearly circular to distinctly elliptical.2PubMed. Ethnic differences in skin physiology, hair follicle morphology and follicular penetration An elliptical follicle produces a flatter ribbon of hair that naturally curves, while a round follicle pushes out a cylindrical strand that hangs straight. So when something alters the geometry of the follicle or the cellular composition of the growing strand, the curl pattern changes with it.
Hormones Are the Most Common Culprit
If your hair changed texture around puberty, pregnancy, postpartum, or menopause, hormones are the most likely explanation. Estrogen, progesterone, testosterone, and thyroid hormones all influence hair follicle behavior, and any significant shift in their levels can remodel how the follicle grows hair.
Puberty is the classic example. Many people recall having fine, straight hair as a child that turned wavy or curly in their early teens. Rising levels of androgens during puberty enlarge follicles and change the composition of the hair strand. The effect can also go the other direction: some people with curly childhood hair find it relaxes into waves during adolescence.
Pregnancy reshuffles the deck again. Elevated estrogen during pregnancy extends the growth phase of hair, which is why pregnant women often notice thicker, more lustrous hair. But the texture can shift too, becoming curlier or straighter depending on the individual. After delivery, the dramatic drop in estrogen causes many of those follicles to enter a resting phase simultaneously, leading to noticeable shedding and sometimes a change in curl pattern as new growth comes in.
Menopause brings its own set of follicle changes. Declining estrogen levels during the menopausal transition can lead to thinning, loss of volume, and changes in hair texture.3PubMed Central. The Menopausal Transition: Is the Hair Follicle “Going through Menopause”? Some women find their hair becomes wiry, coarser, and curlier, while others notice the opposite. The shift is often uneven across the scalp, with new texture showing up most at the hairline and temples first.
Thyroid disorders deserve a mention here too. Both hypothyroidism and hyperthyroidism can alter hair texture, often making it drier and more brittle, which can create the appearance of increased wave or curl. If your texture change came with other symptoms like fatigue, weight changes, or temperature sensitivity, it is worth having your thyroid checked.
Your Genes Set the Range, Not a Fixed Point
Hair curl is not controlled by a single gene with a simple dominant-or-recessive answer. Multiple genes contribute, each with a modest effect, and different populations carry different key variants. In Europeans, variations in the trichohyalin gene (TCHH), which is active in the inner root sheath of the follicle, are strongly associated with straight hair, though the gene explains only about 6% of the total variation in hair shape.4PubMed Central. Common variants in the trichohyalin gene are associated with straight hair in Europeans That leaves a huge portion of the variation unexplained by any single gene.
In East Asian populations, a different gene takes the lead. A genome-wide study of nearly 2,900 Han Chinese individuals found that EDAR was the predominant gene associated with hair straightness, accounting for about 3.7% of total variance. In Uyghur populations, which have mixed European and East Asian ancestry, both EDAR and TCHH play a role, but EDAR has the larger effect.5PubMed. Genome-wide scans reveal variants at EDAR predominantly affecting hair straightness in Han Chinese and Uyghur populations A South African study found yet another set of genetic contributors, including a copper transporter protein called CUTC and the inner root sheath component keratin 74, alongside trichohyalin.6PubMed. The biology and genetics of curly hair
What this means practically is that your hair texture is polygenic: dozens or hundreds of genetic variants each nudge the follicle in a slightly straighter or curlier direction, and their combined effect can shift over time as gene expression changes in response to hormones, aging, or other signals. You were never locked into one texture for life. Your DNA provides a range of possibilities, and your body’s current state determines where within that range you land.
Why Curly Hair Requires More From the Follicle
Making curly hair is biologically harder than making straight hair. A gene expression study comparing very curly and straight hair follicles, plucked from 11 donors, found 85 genes with meaningfully different activity levels between the two types. Of those, 68 were more active in curly follicles and only 17 were more active in straight follicles.7Journal of Advanced Research. Changing human hair fibre colour and shape from the follicle The researchers concluded that curving the hair fiber is a more demanding process for follicle cells than producing a straight one. This asymmetry is part of why texture changes tend to go in both directions: if a follicle’s energy or signaling environment changes, it may no longer sustain the extra effort that curling requires, or it may newly acquire the capacity to do so.
This finding also helps explain why illness, nutritional stress, and aging can all affect curl. Anything that depletes the follicle’s resources or alters its signaling environment can tip the balance between the gene programs that shape straight versus curly fibers.
Medications That Can Flip Your Texture
Some drugs are well documented to change hair texture as a side effect. A systematic review of medications associated with hair texture changes found that the classes most commonly linked to this effect were cancer chemotherapy drugs, anti-seizure medications, retinoids (used for acne and other skin conditions), and immunomodulators.8PubMed. Systemic Medications Associated With Hair Texture Changes
Chemotherapy is the most dramatic example. Many people who lose their hair during cancer treatment find that the hair that regrows is a completely different texture, often curlier and sometimes a different color. This happens because the rapidly dividing cells in the follicle are damaged by the treatment, and when they regenerate, the follicle may rebuild itself with a different shape. The new texture sometimes persists and sometimes reverts over the following year or two as the follicle stabilizes.
Anti-seizure medications like valproic acid and retinoids like isotretinoin can also gradually alter texture during treatment, typically making hair wavier or drier. If you started a new medication around the time your hair changed, that connection is worth exploring with your doctor. The change is usually reversible once the medication is stopped, though it can take several growth cycles for the original texture to return.
The Aging Factor
Even without a hormonal upheaval or new medication, hair texture drifts with age. The follicle undergoes structural changes over the decades as part of general biological aging.9PubMed Central. A Comment on the Science of Hair Aging Follicles gradually miniaturize, producing thinner strands. The ratio of the two cortical cell types within the fiber can shift. Sebum production changes, altering the weight and behavior of hair on the shaft. Melanocyte loss leads to graying, and gray hairs often have a coarser, wirier feel and may sit differently than pigmented hairs did.
People in their 40s and 50s frequently notice that their hair has become wavier or more unruly even in the absence of any obvious trigger. This is the cumulative effect of age-related changes to follicle geometry, strand diameter, and the internal structure of each fiber. The change tends to be gradual enough that many people do not notice until it is well established, then look back and realize the shift has been underway for years.
Can Sun and Environmental Damage Change Your Curl
Sun exposure does not reshape your follicles, but it can alter the properties of existing hair in ways that affect how it looks and behaves. Research exposing hair samples to UV light found widespread changes in the hair’s lipid composition: multiple classes of lipids decreased, including ceramides and triglycerides, while other lipid species increased.10PubMed Central. UV and visible light exposure to hair leads to widespread changes in the hair lipidome Since these lipids help maintain the structural integrity of the hair shaft, their depletion can make hair drier, rougher, and more porous, all of which can make existing waves more pronounced or give straight hair a frizzier, less sleek appearance.
Chemical processing, hard water, chlorine, and heat styling produce similar surface-level damage. These factors do not change the curl programmed into the follicle, but they change how your existing hair expresses that curl. If you have always had a slight wave that was weighed down by heavier, well-moisturized hair, stripping away those lipids and proteins through damage can “reveal” curl that was always there but hidden.
Adjusting Your Routine for New Texture
If your hair has shifted from straight to curly, the products and techniques that worked before likely will not work as well now. Curly and wavy hair has practical differences that go beyond styling preference.
One of the biggest is moisture. In curlier hair, the coiled shape of the strand impairs the distribution of the scalp’s natural oils down the length of the hair, contributing to dryness of both scalp and strand even when sebum production is normal or above average.11PubMed Central. High Curl Pattern Hair and Scalp Care Considerations to Mitigate Seborrheic Dermatitis That is why newly curly hair often feels drier and more frizzy than your old straight hair did. The oil is being produced but cannot travel as effectively along a curved shaft.
Practical adjustments tend to include washing less frequently, using a conditioner with more slip, avoiding sulfate-heavy shampoos that strip oils, and adding leave-in moisture. Brushing dry curly hair disrupts the curl pattern and creates frizz, so most people with curly or wavy hair find that detangling with a wide-tooth comb while the hair is wet and conditioned works better. None of this changes the underlying texture, but it can make a big difference in whether your new curls look defined and healthy or dry and chaotic.
Will Your Old Texture Come Back
It depends entirely on what caused the change. Texture shifts triggered by temporary hormonal events, such as pregnancy and postpartum recovery, often revert partially or fully over the following year or two. Changes caused by a specific medication frequently reverse after the drug is discontinued, though it can take several full hair growth cycles, roughly two to six years, for the complete length of your hair to reflect the new growth pattern.
Texture changes driven by aging or menopause tend to be more permanent because the underlying hormonal shift is not temporary. The follicle has settled into a new state, and without a countervailing hormonal change, there is no signal pushing it back. That said, some people in their 60s and 70s report yet another texture shift as hormones continue to change, so “permanent” is a relative term in the context of hair.
Changes following chemotherapy are unpredictable. Some people’s hair returns to its pre-treatment texture within a year. Others keep the new texture indefinitely. Research has not fully explained why outcomes differ so widely, though the extent of follicle damage and individual genetic variation are likely factors.
When a Texture Change Warrants a Doctor Visit
Most spontaneous texture changes are cosmetic and benign, the result of normal hormonal shifts or aging. But a few scenarios deserve medical attention. A sudden texture change accompanied by significant hair loss or thinning can signal a thyroid disorder, iron deficiency, or an autoimmune condition like alopecia areata. Hair that becomes coarse and brittle along with skin changes and fatigue may point to hypothyroidism. And hair texture changes in children that appear alongside other developmental features can, rarely, indicate a genetic condition affecting connective tissue or keratin formation.
If your texture change was gradual, symmetrical across your scalp, and coincided with a known hormonal event like puberty or menopause, it is almost certainly a normal variation. If it was sudden, patchy, or accompanied by other symptoms, a dermatologist can examine the follicles and run bloodwork to rule out anything that needs treatment. A scalp biopsy is rarely necessary but can provide definitive information about follicle structure if the cause remains unclear.
Why the Genetics of Hair Shape Still Has So Many Gaps
Given how universal and visible hair texture is, you might expect the genetics to be well mapped by now. They are not. The identified genes, including TCHH in Europeans and EDAR in East Asians, collectively explain only a small fraction of the total variation in hair shape across any population. The remaining variation is spread across many genes with individually tiny effects, plus gene-by-gene interactions and gene-by-environment interactions that are difficult to tease apart in studies.
Part of the challenge is that most large genetic studies have been conducted in populations of European descent, leaving the genetics of curl in African, South Asian, and Indigenous populations underexplored. The South African study that identified CUTC and keratin 74 as contributors is one of the few genome-wide scans focused on a non-European, non-East Asian population.6PubMed. The biology and genetics of curly hair Until more diverse populations are studied at scale, the full genetic architecture of curl will remain incomplete. This is not just an academic gap: it limits our understanding of why texture changes happen and why they vary so much between individuals, even within the same family.