Hair loss comes from a surprisingly wide range of sources, but the most common by far is genetics. About 80% of men experience pattern hair loss by age 80, and roughly 40% of women show noticeable thinning by age 50. Beyond inherited traits, hair loss can stem from hormonal shifts, immune system misfires, physical stress on follicles, nutritional gaps, and medical conditions. Understanding which type you’re dealing with is the first step toward knowing whether it’s reversible.
How Hair Grows and Sheds Normally
Every hair on your scalp cycles through four phases: growth, regression, rest, and shedding. The growth phase lasts two to eight years and is when the follicle actively produces a hair shaft. After that, the follicle enters a brief two-week regression period where it shrinks and detaches from its blood supply. Then comes a resting phase, followed by shedding.
At any given time, about 9% of your hair follicles are resting, and it’s normal to lose 100 to 150 hairs per day. Hair loss becomes a problem when something shortens the growth phase, pushes too many follicles into rest at once, or damages follicles so they stop producing hair entirely.
Genetic Pattern Hair Loss
The most widespread cause of hair loss is androgenetic alopecia, commonly called male or female pattern hair loss. It’s driven by a hormone called DHT, which your body produces by converting testosterone through a specific enzyme. DHT binds to receptors on hair follicles in certain areas of the scalp, primarily the crown and temples in men and the part line in women. People with pattern hair loss have higher DHT production and more of these receptors in their balding zones.
When DHT locks onto a follicle, it shortens the growth phase. Over repeated cycles, the follicle produces thinner, shorter, lighter hairs. This process, called miniaturization, is gradual. A thick terminal hair slowly becomes a fine, nearly invisible strand before the follicle eventually stops producing visible hair altogether. The follicle itself doesn’t die right away, which is why early treatment can slow or partially reverse the process.
Hormonal Changes During Menopause
Many women notice significant hair thinning around menopause, and the mechanism is a shift in the ratio between estrogen and androgens. Estrogen helps extend the growth phase of hair and supports blood flow to the scalp. During menopause, estrogen and progesterone drop sharply, while androgen levels decline more gradually. This creates a relative excess of androgens even if their absolute levels aren’t elevated.
Research from 2018 found that postmenopausal women with hair loss had lower estrogen and higher testosterone and DHT compared to postmenopausal women without thinning. Interestingly, most women with female pattern hair loss don’t have abnormally high androgen levels on a blood test. It’s the changed ratio that appears to matter more than any single hormone being out of range. With less estrogen available, scalp blood vessels may also constrict, reducing nutrient delivery to follicles.
When the Immune System Attacks Follicles
Alopecia areata is an autoimmune condition where the body’s own immune cells target hair follicles. Hair follicles normally have a form of immune protection, almost like a shield that keeps them hidden from the immune system. In alopecia areata, that shield breaks down. Certain immune cells then attack the follicles directly, releasing toxic molecules that damage the hair-producing cells.
This attack triggers inflammation and forces follicles out of their growth phase. The result is typically smooth, round patches of hair loss that can appear suddenly, sometimes within days. In some people, the condition remains limited to a few patches. In others, it can progress to total scalp hair loss or even loss of all body hair. Because the follicles aren’t destroyed, just suppressed, regrowth is possible when the immune response calms down.
Stress-Related Shedding
A sudden physical or emotional shock can push large numbers of hair follicles into the resting phase simultaneously. This condition, called telogen effluvium, is one of the most common forms of temporary hair loss. You typically won’t notice it immediately. The shedding starts two to three months after the triggering event, which often makes it hard to connect cause and effect.
Known triggers include high fever, severe infections, major surgery, childbirth, significant psychological stress, crash diets low in protein, thyroid disorders, and stopping birth control pills. Certain medications can also cause it, including some blood pressure drugs, anti-inflammatory painkillers, and antidepressants.
The good news is that telogen effluvium usually resolves on its own. The shedding phase lasts three to six months, and once the underlying trigger is removed, most people see new growth in their affected areas within six to eight months without any treatment.
Thyroid Problems and Hair Loss
Both an overactive and underactive thyroid can cause hair thinning. Thyroid hormones play a direct role in regulating how quickly cells in your hair follicles divide. When thyroid levels are too low, cell division slows down, follicles get stuck in the resting phase, and new growth is delayed. The hair loss from thyroid disorders tends to be diffuse, meaning it thins all over rather than in one spot. Once thyroid levels are corrected, hair typically regrows, though it can take several months.
Nutritional Deficiencies
Your hair follicles are among the most metabolically active structures in your body, and they need a steady supply of specific nutrients. Iron deficiency is one of the most well-documented nutritional causes of hair loss. Dermatologists generally recommend keeping ferritin (your body’s iron storage marker) above 50 to 70 micrograms per liter to support healthy hair growth, which is notably higher than the threshold used to diagnose anemia.
Zinc is another key player. A study of 312 patients with various types of hair loss found statistically lower zinc concentrations compared to healthy controls. Zinc deficiency can show functional effects on hair before blood levels even dip below the standard “normal” range, making it easy to miss on routine testing. Protein, B vitamins, and vitamin D also support the hair cycle, and restrictive or fad diets that cut these nutrients can trigger shedding.
Physical Damage From Hairstyling
Traction alopecia results from repeated pulling on the hair roots over time. It’s most commonly seen in women of African descent who wear tight braids, cornrows, weaves, or extensions, but it also affects ballet dancers, military personnel, Sikh boys who twist their hair, and anyone who regularly wears very tight ponytails or buns.
The condition follows a two-stage pattern. In the early phase, the damage is reversible. If you stop the tight hairstyle, the follicles recover and hair grows back. But chronic, repeated tension leads to inflammation and eventually permanent scarring where the follicle is replaced by fibrous tissue. At that point, the hair loss becomes irreversible. Chemical relaxers and heat styling don’t cause traction alopecia on their own, but they weaken the hair shaft and make it much more vulnerable to breakage when combined with high-tension styles.
How Doctors Identify the Cause
The first thing a clinician determines is whether your hair loss is scarring or non-scarring. In scarring types, the follicle is permanently replaced by scar tissue, and regrowth isn’t possible. In non-scarring types, the follicle is still intact and potentially capable of recovery. A tool called a trichoscope, essentially a magnifying device for the scalp, helps visualize follicle patterns and can guide where to look more closely.
When the diagnosis isn’t clear from a physical exam, a small scalp biopsy can provide definitive answers. Under a microscope, scarring alopecia shows fibrous tissue where follicles used to be and loss of the oil glands that normally sit beside each follicle. Non-scarring types like pattern hair loss or telogen effluvium show follicle miniaturization or an abnormal ratio of growing to resting hairs, but the basic follicle structure remains. Blood tests for thyroid function, iron, zinc, and hormone levels help rule out treatable underlying causes.