Why Did My Hair Become Curly? The Science Explained

Hair that was once stick-straight can gradually or suddenly develop waves, kinks, or full curls, and the change is not in your imagination. The shape of each strand is set inside the follicle before the hair ever emerges from your scalp, so when texture changes, something has altered the follicle itself or the internal chemistry of the fiber it produces. Hormonal shifts, medications, thyroid conditions, aging, and even changes to the physical geometry of the follicle can all drive a transition from straight to curly, and the science behind each trigger is more interesting than a simple “it’s genetic.”

What Makes Hair Curl in the First Place

Whether a strand grows straight or curly comes down to two things working together: the shape of the follicle it grows from and the distribution of structural proteins inside the strand. A perfectly round follicle that sits straight in the skin tends to produce straight hair. A follicle that is curved, hooked, or angled pushes the emerging fiber into a bend, and the more asymmetric the follicle, the tighter the curl. Research comparing hair types globally has found that straighter fibers tend to have a circular cross-section with a larger area, while curlier fibers are more elliptical with a smaller cross-section.¹ That elliptical shape creates uneven tension across the strand as it hardens, which reinforces the curl pattern.

Inside the strand, the structural protein keratin forms long chains held together by chemical bonds, most critically disulfide bonds. These bonds act like tiny bridges between neighboring protein chains. When the bridges form symmetrically all the way around, the strand stays straight. When they form unevenly, with more bonds on one side of the strand than the other, the strand bends. This asymmetry can be set genetically, but it can also be disrupted by anything that alters the biochemical environment inside the follicle. Understanding this dual mechanism, follicle geometry plus internal protein bonding, is key to understanding why your hair might change.

Hormones Are the Most Common Trigger

If your hair changed texture around puberty, pregnancy, or menopause, hormones are the most likely explanation. Androgens like testosterone and dihydrotestosterone (DHT) directly affect hair follicle size, activity, and shape. During puberty, rising androgen levels can reshape follicles, turning fine, straight childhood hair into thicker, wavier adult hair. The change sometimes happens gradually over years, which is why many people don’t notice it until they look back at old photos.

Pregnancy is another well-known trigger. The surge in estrogen during pregnancy extends the growth phase of each strand, meaning fewer hairs are shed and individual strands grow longer than usual. But estrogen and progesterone can also alter the follicle’s curvature and the way keratin proteins are deposited within the strand. Some women find their hair becomes curlier during pregnancy; others find it goes straighter. After delivery, when hormone levels drop sharply, the texture often shifts again, sometimes returning to the original pattern, sometimes settling into something new.

Menopause brings its own version of this story. Declining estrogen and a relative increase in androgens can thin the hair, change its texture, and alter its curl pattern. Men experience a parallel hormonal drift as testosterone levels gradually decline with age, which can shift hair texture in either direction. The common thread is that follicles are hormone-sensitive organs: when the hormonal environment changes, the follicle’s output changes with it.

Thyroid Conditions and Other Medical Causes

A change in hair texture that arrives alongside fatigue, weight changes, or dry skin may point to a thyroid problem. Hypothyroidism, where the thyroid gland produces too little hormone, is known to cause dry, thinning, and coarse hair because hair follicles are direct targets of thyroid hormones. The thyroid hormones T3 and T4 influence multiple aspects of hair biology: they help keep follicles in their active growth phase, support pigmentation, and regulate the turnover of the cells that build the hair shaft.² When those hormones are out of balance, the resulting changes to follicle behavior can make previously smooth hair become wiry or wavy.

Autoimmune conditions beyond thyroid disease can also affect hair texture. Alopecia areata, for instance, sometimes causes hair to regrow with a completely different curl pattern after a period of loss. Chronic illness, major surgery, or severe stress can push large numbers of follicles into a resting phase simultaneously. When those follicles reactivate and new hair grows in, the strands may differ in color and curl from the hair that fell out, because the follicle essentially “reboots” under different physiological conditions.

Medications That Change Hair Texture

Certain drugs are known to alter hair curl. Retinoids, a class of vitamin-A-derived medications used for skin conditions, are one example. Clinical case reports have documented hair curling in patients taking alitretinoin, a retinoid used for severe hand eczema.² The mechanism appears related to how retinoids influence keratinocyte differentiation, the process by which cells in the follicle mature and produce the proteins that form the hair shaft. If that maturation process is altered, the internal structure of the strand changes, and with it the curl pattern.

Chemotherapy is another common culprit. Many cancer patients find that hair growing back after chemotherapy has a dramatically different texture, often curlier than before treatment. This so-called “chemo curl” happens because the powerful drugs damage rapidly dividing cells in the follicle. When the follicle recovers, it may rebuild with a different shape or produce keratin with a different bonding pattern. For some people, chemo curl is permanent; for others, the original texture gradually returns over a year or two as the follicle fully recovers.

Hormonal medications, including oral contraceptives and hormone replacement therapy, can also shift texture. Because these drugs alter the balance of estrogen, progesterone, and sometimes androgens circulating in the body, they can have the same kinds of effects on follicles as the natural hormonal transitions discussed earlier. Stopping a long-term hormonal medication sometimes triggers a texture change that catches people off guard.

The Role of Genetics

Hair curl has a strong genetic basis, but it is not controlled by a single gene. Genome-wide association studies have identified links between curl and variation in several genes, including trichohyalin (a protein that helps shape the inner root sheath of the follicle), a copper transporter protein called CUTC, and keratin 74, a structural component of the inner root sheath.³ These genes influence follicle geometry and the way keratin fibers are assembled, which in turn determines how much the emerging strand bends.

The involvement of multiple genes explains why hair texture can shift across a person’s lifetime even without an obvious environmental trigger. Genes are not static switches; their activity levels change in response to aging, hormonal signals, and other biological cues. A gene variant that had minimal effect during childhood might become more influential once puberty alters the hormonal landscape. This is also why siblings raised in the same household can have noticeably different curl patterns, and why your own texture at thirty might look nothing like it did at fifteen.

Population-level studies highlight the diversity. While the most common classification divides hair into broad groups by ancestry, this framework doesn’t capture the enormous variability within any single group. A more detailed global classification has proposed eight distinct curl types, though even that system struggles with reproducibility because hair texture exists on a continuous spectrum rather than in neat categories.¹ The practical takeaway is that your genetics set a range of possible textures, and your body’s current state determines where within that range your hair lands at any given time.

The Chemistry of Curl at the Molecular Level

The disulfide bonds inside a hair strand are worth understanding a bit further, because they explain both natural curl changes and why certain chemical treatments work. Each strand of hair contains long keratin chains arranged in bundles. Sulfur-containing amino acids on neighboring chains can form disulfide bridges, essentially locking the chains into a particular spatial arrangement. The more disulfide bridges that form asymmetrically across the strand, the more it curves.

This is exactly the chemistry exploited by a salon perm. In the classic permanent-wave process, a reducing agent first breaks existing disulfide bonds throughout the strand. The hair is then wrapped around rods to impose a new shape, and an oxidizing agent is applied to re-form the disulfide bonds in their new positions. Research has confirmed that this two-stage reduction and oxidation process relies on sulfhydryl-disulfide interchange as the key bond-exchange reaction.⁴ The same underlying chemistry explains why heat, humidity, and pH shifts can temporarily alter curl: they loosen or rearrange hydrogen bonds (weaker than disulfide bonds but still significant), allowing the strand to shift shape until it dries or cools again.

When your body’s internal chemistry changes through illness, medication, or hormonal flux, it can alter the way disulfide and hydrogen bonds form during the keratinization process inside the follicle. The hair that emerges carries those changes baked in, which is why a texture shift from an internal cause tends to be consistent along the entire new-growth portion of the strand rather than appearing patchily along already-grown hair.

Aging and Gradual Texture Shifts

Aging brings a slow but real change to hair texture for most people, and it goes beyond just going gray. As you get older, hair follicles shrink. The strands they produce get thinner, and thinner strands are more easily influenced by even slight asymmetries in protein distribution. Someone whose follicles always had a mild asymmetry might not have noticed any curl when their hair was thick and heavy enough to weigh itself straight. As the strands thin with age, the curl that was always encoded in the follicle’s geometry can finally express itself.

Sebum production also declines with age. Sebum coats the hair strand and adds weight, which can suppress curl. Less sebum means lighter, drier hair that springs up more readily. Meanwhile, the cumulative effects of sun exposure, heat styling, and chemical treatments can damage the cuticle layer of the strand, changing how it holds moisture and how rigid it remains. All these factors can conspire to make hair that was reliably straight for decades begin to develop a wave or curl.

Gray and white hairs sometimes have a different texture from pigmented hairs growing on the same scalp. Melanocytes, the cells that produce pigment, sit in the follicle and interact with the keratinocytes that build the strand. When melanocytes stop functioning and the hair grows in without pigment, the strand’s internal structure can shift subtly. Many people notice that their gray hairs are coarser and more wiry than their pigmented ones, which can contribute to an overall change in the feel and behavior of the hair.

Is the Change Permanent

It depends entirely on the cause. Texture changes driven by a reversible trigger, like a medication, a temporary hormonal shift such as pregnancy, or a treatable thyroid condition, often reverse once the trigger is removed or treated. The catch is that hair grows slowly, typically around half an inch per month, so even after the underlying cause is resolved, you’ll need to wait for the entire length of your hair to cycle through before you see the old texture fully return.

Texture changes driven by aging or permanent hormonal shifts like menopause tend to stick around. The follicle has remodeled, and without a new trigger to remodel it again, the new curl pattern is the new normal. Post-chemotherapy texture changes fall somewhere in between: many people see their original texture gradually return over one to three years, but a meaningful minority find the change permanent.

If your hair changed and you’re unsure why, it’s worth considering what else was happening in your body at the time. Starting or stopping a medication, a major illness, significant weight loss or gain, a period of extreme stress, or a new diagnosis can all be clues. For most people a texture change is cosmetically surprising but medically benign. However, if the change arrives with other symptoms like hair thinning, fatigue, skin changes, or unexplained weight shifts, a visit to your doctor is a reasonable step. Thyroid panels and basic blood work can rule out some of the more common medical explanations relatively quickly.

Cross-Sectional Shape and Why Ethnicity Is Only Part of the Story

You may have heard that hair type is determined by ethnic background. There’s a kernel of truth there, but the full picture is far more nuanced. A large multinational study examining over 19,000 individuals noted that Asian hair tends to have the thickest cuticle layer and the largest diameter, contributing to its characteristic straightness.⁵ But within every population, there is enormous individual variation. People of East Asian descent can have wavy or curly hair; people of West African descent can have relatively loose curl patterns.

The cross-sectional geometry of the strand, round versus elliptical, is a better predictor of curl than ancestry alone. And that geometry can change over a lifetime for all the reasons covered above. So when someone asks “why did my hair become curly,” the honest answer usually involves an interplay between the genetic range they inherited and whatever biological event shifted their follicles along that range. Ethnicity sets the starting point and the boundaries, but hormones, health, and aging determine where within those boundaries you land at any given moment. Two people with nearly identical genetic backgrounds can have quite different hair at the same age if their hormonal histories diverge.

Humidity, Climate, and the Illusion of New Curl

Sometimes what feels like a permanent texture change is actually a response to your environment. Hair is hygroscopic, meaning it absorbs moisture from the air. When hydrogen bonds in the strand absorb water, they break and reform in new positions, which can cause straight hair to wave and wavy hair to frizz into tighter curls. If you’ve moved to a more humid climate, or if seasonal humidity has increased, you might be experiencing an environmental curl that reverses in drier conditions.

This is distinct from a true follicle-level change. Environmental curl affects the already-grown strand and fluctuates day to day. A genuine texture change from hormones, health, or aging shows up consistently in new growth from the root and doesn’t wash out when the weather changes. If you’re trying to figure out which category you fall into, pay attention to new growth close to the scalp after washing and air-drying in a controlled indoor environment. If the new growth curls even in low humidity, the change is probably coming from inside the follicle rather than from the weather.

Heat damage and chemical processing can also mimic a texture change. Repeated flat-ironing can break internal bonds and alter the curl pattern of existing strands, though the new growth from the follicle will still reflect whatever the follicle is currently programmed to produce. Conversely, stopping years of regular straightening can reveal a natural texture you didn’t know you had, which can feel like a sudden change even though the follicle has been producing that curl all along.