Can My Straight Hair Turn Curly?

Straight hair can absolutely turn curly, and it happens to people more often than you might expect. Hormonal shifts during puberty, pregnancy, and menopause are among the most common triggers, but medications, thyroid conditions, and even autoimmune diseases can also change hair texture. The explanation lies in a surprisingly complex interplay between follicle shape, protein chemistry, and the signals your body sends to growing hair cells.

What Determines Whether Hair Is Straight or Curly

The shape of your hair depends largely on the shape of the follicle it grows from. A round, symmetrical follicle produces a round fiber that grows straight. An oval or asymmetrical follicle produces a fiber with an uneven cross-section that bends as it emerges, creating curl. Researchers have built models showing how the internal structure and curvature of the fiber interact at multiple levels, from the molecular bonds holding proteins together all the way up to the visible curl pattern you see in the mirror.¹

Inside every strand of hair, a tough protein called keratin forms long chains held together by two types of bonds. Hydrogen bonds are weak and break easily with water, which is why your hair changes shape when it gets wet and resets when it dries. Disulfide bonds are much stronger, acting as the permanent scaffolding that locks in your hair’s resting shape. These disulfide bonds link keratin chains to each other and to surrounding matrix proteins, and their arrangement determines whether a strand curls tightly, waves gently, or hangs perfectly straight.²

When something changes the shape of the follicle or disrupts how keratin and its associated proteins assemble, the hair that grows out can have a very different texture than the hair that came before it. This is why texture changes tend to appear gradually: your existing strands stay the same, but the new growth coming in may be noticeably different.

Hormonal Shifts Are the Most Common Trigger

If you have heard stories about someone’s hair going from stick-straight to wavy after puberty or pregnancy, hormones are almost certainly the explanation. The hair cycle and the structure of the hair follicle are highly affected by various hormones, including androgens, estrogens, and thyroid hormones.³ These hormones influence how fast hair grows, how long the growth phase lasts, and critically, the shape and behavior of the follicle itself.

Puberty brings a surge of androgens that can reshape follicles, which is why many people notice their hair texture changing in their teens. Pregnancy floods the body with estrogen and progesterone, and the hair that grows during or after pregnancy may come in with a distinctly different pattern. Menopause involves a steep drop in estrogen, and many women report their hair becoming wavier or even curly during this transition. The common thread is that significant hormonal upheaval can physically alter follicle geometry, and the effect may be temporary or permanent depending on the individual.

Aging itself may play a role independent of specific hormonal events. As you get older, follicles can change shape gradually, the diameter of individual strands may shift, and the balance of proteins being produced inside the follicle can drift. Some people find their hair gets straighter with age; others find it gets curlier. There is no single predictable pattern, which frustrates anyone hoping for a clean rule.

Thyroid Disorders and Hair Texture

Your thyroid gland produces hormones that regulate metabolism throughout the body, and hair follicles are sensitive to those signals. A morphological study comparing scalp hair across people with normal thyroid function, hyperthyroidism, and hypothyroidism found that straight hair was common in people with normal thyroid levels, while both overactive and underactive thyroid conditions were associated with a greater occurrence of curly hair.⁴

This is worth knowing because thyroid conditions are common, affecting millions of people, and the texture change may be one of the earliest noticeable symptoms. If your hair seems to be changing and you also have symptoms like unexplained fatigue, weight changes, or feeling unusually hot or cold, it is worth talking to a doctor about thyroid testing. The hair texture shift may partially reverse once thyroid levels are brought under control with treatment, though that is not guaranteed.

Medications That Change Hair Texture

Certain drugs are well documented to alter hair texture, sometimes dramatically. A review of reported cases found that texture changes were primarily associated with cancer chemotherapy drugs, antiepileptic medications, retinoids, immunomodulators, and antiretroviral therapy.⁵ Chemotherapy agents were the most frequently reported culprits, followed by antiepileptics and then retinoids.

Chemotherapy is a particularly dramatic case. Many of these drugs damage the rapidly dividing cells in the hair matrix, which can cause hair to fall out entirely. When hair regrows after treatment ends, the regenerating follicle may produce a fiber with a completely different curl pattern, color, or both. The hair-regrowth pattern depends on how the drug disrupted the growth cycle and damaged follicle cells.⁶ People who had straight hair before chemo sometimes find their regrowth comes in curly, and it can stay that way for months or even permanently. The phenomenon is common enough among cancer survivors that it has its own informal name: “chemo curls.”

Antiepileptic drugs like valproic acid and retinoids used for severe acne or other skin conditions can also shift hair texture, though the changes tend to be less extreme than what chemotherapy produces. If you notice your hair texture changing after starting a new medication, it is a side effect worth mentioning to your prescriber, even if it is not dangerous.

Hair Regrowth After Alopecia Areata

Alopecia areata is an autoimmune condition in which the immune system attacks hair follicles, causing patches of hair loss. When hair grows back in affected areas, it sometimes returns with a different color or texture. A case report documented that while pigment changes are well recognized during regrowth, a change in hair texture from straight to curly (or the reverse) has been reported but is considered rare.⁷

The mechanism is not fully understood, but the thinking is that the immune attack damages or remodels parts of the follicle responsible for shaping the hair shaft. When the follicle recovers and begins producing hair again, its geometry may have shifted enough to change the curl pattern. Some people see their original texture return over time as the follicle continues to heal and normalize; others find the change persists. The white or lighter-colored hair that often appears first during regrowth may also have a different texture than the pigmented hair that eventually replaces it.

The Genetics Are More Complex Than You Would Expect

Hair shape is not a single-gene trait. Genome-wide studies have identified multiple genes involved, and which genes matter most depends heavily on your ancestry. In people of European descent, common variants in the trichohyalin gene (TCHH) show the strongest association with hair shape, explaining roughly 6% of the total variation.⁸ Trichohyalin is a structural protein expressed in the inner root sheath of the hair follicle, the sleeve that molds the growing hair fiber as it moves toward the skin surface.

In East Asian populations, a different gene dominates. A genome-wide scan of nearly 3,000 Han Chinese individuals found that a variant in the EDAR gene was the predominant factor, accounting for about 4% of variance in hair straightness.⁹ In Uyghur populations, which have mixed European and East Asian ancestry, both EDAR and TCHH play a role, but EDAR has the larger effect.⁹ A separate meta-analysis confirmed that TCHH-related gene variants showed the largest effect on hair shape in European cohorts, while EDAR was predominant in East Asians.¹⁰

The practical implication is that hair shape is genuinely polygenic, meaning many genes contribute small effects. Even the biggest single-gene contributors explain only a fraction of the variation, with the rest spread across at least eight other identified loci and likely many more that have not been mapped yet. This complexity helps explain why hair texture can shift over a lifetime: you carry all of these gene variants from birth, but their expression can be influenced by hormonal and environmental signals that change as you age. A gene variant that was not doing much during childhood could become more active under the hormonal conditions of adulthood, subtly reshaping the follicle.

Humidity, Water, and Temporary Curl

If you have noticed your hair getting frizzy or wavy in humid weather and returning to its usual state in dry air, that is a different phenomenon from the permanent texture changes discussed above. Hair fibers act as shape-memory materials that respond to water. When hair absorbs moisture, the hydrogen bonds inside the fiber break apart, allowing the protein chains to shift and the strand to swell and flex. When the hair dries, new hydrogen bonds form in whatever configuration the strand happens to be in, locking in a temporary shape.¹¹

This is the same principle behind blow-drying with a round brush or using curling irons: you are breaking hydrogen bonds with heat or water, reshaping the strand, and then letting new hydrogen bonds form to hold the new shape. It is also why these styles wash out. The stronger disulfide bonds remain untouched, and the hair eventually returns to its genetically programmed shape. Chemical treatments like perms and relaxers work by breaking and reforming disulfide bonds, which is why their effects are permanent until new hair grows in.

The temporary effects of humidity are more pronounced in hair that already has some wave or irregularity to its cross-section. Truly round, uniformly structured straight hair resists humidity-driven frizz better than hair with even slight asymmetries. So if you feel like your “straight” hair is getting curlier in humid climates, your strands may have always had a subtle wave potential that only becomes visible under the right moisture conditions.

Uncombable Hair Syndrome and Other Rare Genetic Conditions

On the more unusual end of the spectrum, certain genetic conditions directly affect the structural proteins involved in shaping hair fibers. Uncombable hair syndrome (UHS) is a rare condition that typically appears in children and produces dry, frizzy, straw-colored hair that stands out from the scalp and resists all attempts at combing or brushing.¹²

Researchers identified that UHS is caused by recessive mutations in three genes: PADI3, TGM3, and TCHH. All three are involved in building and modifying proteins during hair shaft formation. The two enzymes (PADI3 and TGM3) chemically modify the structural protein trichohyalin (TCHH), and when any one of these three components is defective, the hair shaft develops an abnormal triangular or kidney-shaped cross-section instead of the normal round or oval shape.¹³ The good news is that UHS tends to improve on its own as children grow older, and it does not affect overall health.

UHS is interesting in this context because it involves the same TCHH gene that genome-wide studies have linked to normal variation in hair curliness. The difference is one of degree: common TCHH variants nudge hair toward being slightly straighter or curlier, while the recessive mutations in UHS cause a more dramatic disruption to hair shaft structure. It is a useful reminder that the same molecular machinery responsible for everyday differences in hair shape can produce striking results when something goes significantly wrong.

When to Be Concerned and When to Relax

Most hair texture changes are benign. If your hair has gradually shifted from straight to wavy over a few years, especially during a major life stage like puberty, pregnancy, or perimenopause, you are experiencing something well within the range of normal. The change is real, not imagined, and it is driven by biological processes that are altering your follicles.

A few scenarios do warrant a conversation with a doctor. A sudden or dramatic texture change accompanied by other symptoms, like fatigue, weight fluctuation, or skin changes, could point to a thyroid disorder. Hair that comes back with a different texture after falling out in patches may indicate alopecia areata. And texture shifts that coincide with starting a new medication are worth reporting, even if the change itself is not harmful, because it tells your provider something about how the drug is affecting you.

One common misconception is that shaving your head or cutting your hair short will change its texture. It will not. Cutting hair has no effect on the follicle, and since the follicle determines texture, the regrowth will be the same. The illusion of thicker or coarser hair after shaving comes from the blunt-cut end of the new growth, which feels stiffer than a naturally tapered tip. Another misconception is that using certain shampoos or supplements can permanently change your curl pattern. No topical product reaches the follicle in a way that would reshape its geometry. Products can temporarily alter the appearance and feel of your hair, but the underlying texture will remain whatever your follicles are producing.

People also sometimes wonder whether they can intentionally make their straight hair turn curly without chemical treatments. There is no evidence that any lifestyle change, diet, or supplement can reliably shift follicle shape. The hormonal and genetic drivers of hair texture are largely outside your conscious control. You can use heat styling, wet sets, or chemical perms to create temporary or semi-permanent curl, but your follicles will keep producing the same fiber they were already making. The science here is clear but somewhat unsatisfying: your body decides, and you adapt.