Hair loss has dozens of possible causes, ranging from genetics and hormonal shifts to nutritional gaps and everyday styling habits. Losing between 50 and 150 hairs per day is completely normal, so finding strands on your pillow or in the shower drain doesn’t necessarily signal a problem. But when shedding increases noticeably or hair starts thinning in specific areas, something deeper is usually at play. Here are the most common reasons it happens.
Genetic Pattern Thinning
The single most common cause of hair loss is inherited sensitivity to a hormone called dihydrotestosterone, or DHT. Your body naturally converts testosterone into DHT, and in people with a genetic predisposition, DHT binds to receptors on hair follicles and gradually shrinks them. Over time, each growth cycle produces a thinner, shorter strand until the follicle stops producing visible hair altogether. This process is called follicle miniaturization, and it’s the hallmark of pattern baldness in both men and women.
The numbers are striking: by age 35, roughly two-thirds of American men have some degree of noticeable hair loss, and by 50, about 85% have significantly thinning hair. Women experience the same underlying mechanism but typically see diffuse thinning across the top of the scalp rather than a receding hairline. Because DHT drives the process, treatments that block its production or prevent it from binding to follicles can slow or partially reverse the thinning, especially when started early.
Stress, Illness, and Sudden Shedding
If your hair seems to fall out in clumps two or three months after a major physical or emotional event, you’re likely experiencing a condition called telogen effluvium. This happens when a large percentage of your hair follicles are pushed into their resting phase all at once, then release their strands simultaneously a few months later. It can look alarming, with handfuls of hair coming out during washing or brushing, but it’s almost always temporary.
Common triggers include high fever, severe infections, major surgery, significant psychological stress, childbirth, and crash diets low in protein. Certain medications can also set it off, including beta-blockers, some antidepressants, anti-inflammatory drugs, and retinoids used for skin conditions. After the triggering event resolves, the heavy shedding phase typically lasts three to six months before new growth fills back in. No special treatment is usually needed beyond addressing the original cause and giving your body time to recover.
Hormonal Shifts
Hormones regulate the hair growth cycle, and any significant swing can disrupt it. Postpartum hair loss is one of the most recognizable examples. During pregnancy, elevated estrogen levels keep hair in its active growth phase longer than usual, which is why many pregnant women notice thicker, fuller hair. After delivery, estrogen drops sharply, and a large wave of follicles enters the resting phase simultaneously. A few months later, all that “extra” hair sheds at once. It typically resolves on its own within a year.
Stopping or starting hormonal birth control can trigger a similar shift. Menopause brings a longer-term decline in estrogen that may cause gradual, diffuse thinning. Polycystic ovary syndrome, which raises androgen levels in women, can also lead to thinning that mirrors the DHT-driven pattern seen in men.
Thyroid Problems
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can cause noticeable hair loss. Thyroid hormones help regulate metabolism throughout the body, including inside hair follicles. When production swings too high or too low, follicles may stop cycling properly and shift into their resting phase prematurely. The result is diffuse thinning across the entire scalp rather than bald patches.
You might notice excessive shedding during washing, brushing, or styling. The good news is that thyroid-related hair loss is usually reversible once your thyroid levels are brought back into a normal range. If you’re losing hair alongside other symptoms like unexplained weight changes, fatigue, or sensitivity to cold or heat, a simple blood test can check your thyroid function.
Nutritional Deficiencies
Your hair follicles are metabolically active and depend on a steady supply of nutrients. Iron deficiency is one of the best-studied nutritional causes of hair shedding. Interestingly, you don’t need to be fully anemic to experience it. Research suggests that ferritin (your body’s iron storage protein) levels below 70 ng/mL may be too low to sustain a normal hair cycle, even if your overall blood counts look fine. This “nonanemic iron deficiency” is easy to miss on routine bloodwork if your doctor isn’t specifically checking ferritin.
Other nutritional shortfalls linked to hair loss include low vitamin D, zinc, biotin, and protein. Crash diets and highly restrictive eating patterns are frequent culprits because they deprive follicles of the building blocks they need. If your hair loss started after a significant dietary change, a blood panel checking iron stores, vitamin D, and other key nutrients is a reasonable first step.
Medications That Cause Shedding
A number of prescription medications list hair loss as a side effect, and it’s more common than many people realize. Retinoids used for severe acne or psoriasis are among the most frequent offenders. In one study of patients taking a retinoid for psoriasis, more than 63% experienced hair loss at higher doses, though it reversed after stopping the medication.
Valproic acid, a mood stabilizer used for bipolar disorder and seizures, causes hair loss in roughly 11% of people who take it. Certain antifungal medications can also trigger diffuse shedding. In one survey, nearly 80% of patients taking a particular antifungal long-term reported scalp hair loss, with shedding beginning about two to three months into treatment. Beta-blockers, calcium channel blockers, blood thinners, and some antidepressants round out the list. In most cases, this type of shedding is reversible once the medication is adjusted or discontinued, though it can take several months for regrowth to become visible.
Hairstyles and Physical Damage
Sustained tension on hair follicles can cause a type of hair loss called traction alopecia. This is especially common with tight cornrows, locs, braids, buns, ponytails pulled snugly, and hair extensions or weaves applied to chemically relaxed hair. Even wearing a hat, head scarf, or other covering over tightly pulled hair can contribute over time. The constant tugging irritates follicles and eventually damages them.
Early warning signs include pain or stinging at the scalp, crusting, and a “tenting” effect where sections of skin are visibly pulled upward. At this stage, the damage is reversible if you switch to a looser style. A simple rule from the American Academy of Dermatology: if your hairstyle hurts, it’s too tight. But if pulling continues long enough, the follicles scar over and produce smooth, shiny bald patches where hair will no longer grow back. Catching it early makes all the difference.
Autoimmune Conditions
Alopecia areata is an autoimmune disorder in which the immune system mistakenly attacks hair follicles, causing smooth, round bald patches that can appear suddenly on the scalp, beard, eyebrows, or elsewhere. It can strike at any age and often begins in childhood or early adulthood. In some cases, hair regrows on its own within months. In others, the patches expand or recur over time. Unlike pattern baldness, alopecia areata doesn’t destroy the follicle permanently, so regrowth remains possible even after extended periods of hair loss.
Lupus is another autoimmune condition that can cause hair thinning, sometimes accompanied by scarring on the scalp that makes the loss permanent in affected areas. If you notice sudden, well-defined bald patches or hair loss alongside joint pain, rashes, or fatigue, an autoimmune cause is worth investigating.
Scalp Conditions and Infections
Fungal infections like ringworm of the scalp can cause patchy hair loss, often with redness, scaling, or broken-off hair shafts at the skin surface. It’s most common in children but can affect adults too. Severe dandruff or seborrheic dermatitis doesn’t directly cause hair to fall out, but the persistent inflammation and scratching can weaken strands and accelerate shedding. Folliculitis, an infection of individual hair follicles, can also damage them if it becomes chronic or deep-seated. These causes are generally treatable once correctly identified.