Hair loss has dozens of possible causes, ranging from genetics and hormonal shifts to stress, nutritional gaps, and medication side effects. Most people lose 50 to 100 hairs a day as part of normal cycling, but when shedding increases noticeably or hair stops growing back, something specific is usually driving it. Here’s a breakdown of the most common culprits.
Genetics and Hormonal Sensitivity
The single most common cause of hair loss is androgenetic alopecia, often called male- or female-pattern hair loss. It affects roughly half of men by age 50 and a significant number of women after menopause. The mechanism centers on a hormone called DHT (dihydrotestosterone), which is converted from testosterone in the body. DHT binds to receptors on genetically susceptible hair follicles, causing them to shrink over time. Each growth cycle produces a thinner, shorter strand until the follicle eventually stops producing visible hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it tends to appear as a widening part and overall thinning across the top of the scalp, usually without a receding hairline. Because it’s driven by genetics and hormones rather than damage, it progresses gradually over years and doesn’t reverse on its own.
Stress-Related Shedding
A condition called telogen effluvium is responsible for the dramatic, diffuse hair shedding many people experience after a major physical or emotional stressor. Normally, only about 10% of your hair is in the resting (telogen) phase at any given time. After a significant trigger, up to 70% of actively growing hairs can prematurely shift into that resting phase, then fall out in clumps weeks later.
The timeline is what catches people off guard. Hair loss typically appears two to three months after the triggering event, which means you might not connect the shedding to its actual cause. Common triggers include surgery, high fever, severe illness, rapid weight loss, and intense emotional stress. Acute telogen effluvium usually resolves within six months, and hair density returns to normal once the follicles re-enter their growth phase. Chronic cases lasting longer than six months can occur when the underlying stressor persists.
Thyroid Disorders
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can cause hair loss. Thyroid hormones regulate the growth cycle of hair follicles, so when levels swing too high or too low, follicles may stop producing new hair. The loss is usually diffuse, meaning it happens all over the scalp rather than in patches.
Beyond shedding, thyroid imbalances often change the texture of your hair. Strands may feel dry, coarse, and more prone to breakage. This texture change can be an early clue, sometimes appearing before noticeable thinning. The good news is that thyroid-related hair loss is typically reversible once hormone levels are brought back into balance with treatment, though regrowth can take several months.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition in which the immune system mistakenly attacks hair follicles. The hallmark sign is smooth, round bald patches that appear suddenly, often on the scalp but sometimes in the beard, eyebrows, or elsewhere on the body. The condition has several variants: patchy (the most common), diffuse thinning, alopecia totalis (complete scalp hair loss), and alopecia universalis (loss of all body hair).
The underlying process involves a breakdown of the hair follicle’s “immune privilege,” a protective barrier that normally shields follicles from immune cell attack. When that protection fails, inflammatory cells swarm the follicle and shut down hair production. Alopecia areata can appear at any age and is unpredictable. Many people experience regrowth within a year, but the condition can recur.
Nutritional Deficiencies
Your hair follicles are among the fastest-dividing cells in the body, making them highly sensitive to nutritional shortfalls. Iron deficiency is one of the most well-documented nutritional causes of hair loss. Studies show that women with diffuse, nonscarring hair loss have significantly lower iron stores (measured as serum ferritin) than those without hair loss. In one study, women with hair shedding had average ferritin levels around 16 ng/mL compared to 60 ng/mL in controls. When ferritin drops to 30 ng/mL or below, the odds of developing telogen effluvium increase dramatically, roughly 21 times higher than in people with adequate levels.
Iron isn’t the only nutrient that matters. Deficiencies in zinc, biotin, vitamin D, and protein can all contribute to thinning. Crash diets and restrictive eating patterns are a common trigger because they deprive follicles of the raw materials they need. If you’re losing hair and suspect a nutritional cause, a blood test checking ferritin, vitamin D, and zinc levels can point you in the right direction.
Hormonal Shifts After Pregnancy
Postpartum hair loss is startlingly common and almost always temporary. During pregnancy, elevated estrogen levels keep more hair in the growth phase than usual, which is why many women notice thicker, fuller hair while pregnant. After delivery, those hormone levels drop rapidly, and all the hair that was “held in place” enters the shedding phase at once.
This shedding usually starts about three months after giving birth and can be dramatic, with handfuls of hair coming out in the shower or on your pillow. It typically resolves between 6 and 12 months postpartum as hormone levels stabilize and follicles cycle back into active growth. No treatment is necessary in most cases, though the volume of shedding can be alarming if you’re not expecting it.
Medications That Trigger Shedding
A wide range of common medications can cause hair shedding as a side effect. The mechanism is usually the same as stress-related shedding: the drug pushes a larger-than-normal percentage of follicles into the resting phase. Drug classes documented to cause this include retinoids (used for acne and skin conditions), certain antifungal medications, mood stabilizers, blood thinners (anticoagulants), and blood pressure medications (antihypertensives).
Chemotherapy drugs work differently. Rather than shifting hair into a resting phase, they directly attack rapidly dividing cells, which includes hair follicle cells. This causes a faster, more severe type of hair loss called anagen effluvium, which can begin within weeks of starting treatment. Unlike medication-induced telogen effluvium, which produces gradual thinning, chemotherapy-related loss tends to be sudden and extensive. In both cases, hair typically regrows after stopping the medication.
Physical Damage and Traction
Hairstyles that pull on the hair, including tight ponytails, braids, cornrows, and extensions, can cause traction alopecia. The constant tension damages follicles along the hairline and wherever the pull is strongest. In its early stages, traction alopecia is fully reversible: change the hairstyle, reduce the tension, and the hair grows back.
Left untreated over years, however, the damage becomes permanent. Warning signs that traction alopecia has progressed to scarring include smooth, shiny patches on the scalp where hair once grew, extensive bald areas, and a complete lack of new growth in affected zones. Once scarring develops, the follicles are destroyed and cannot regenerate. This is why early intervention matters so much. Heat styling, chemical relaxers, and harsh coloring treatments compound the problem by weakening strands that are already under mechanical stress.
Scalp Conditions and Inflammation
Chronic scalp inflammation can disrupt hair growth even when the follicles themselves are healthy. Seborrheic dermatitis, the condition behind persistent dandruff and flaky, itchy scalp, is a good example. Excess oil production on the scalp creates irritation and inflammation, which triggers intense itching. Scratching damages follicles and interrupts normal growth cycles.
A yeast called Malassezia, which naturally lives on the skin, plays a role as well. When oil production spikes, Malassezia overgrows, fueling further inflammation and follicle damage. The resulting hair loss is temporary. Once the inflammation is controlled through antifungal and anti-inflammatory treatments, hair regrows normally. Other scalp conditions that can cause hair loss include psoriasis, fungal infections (ringworm), and folliculitis, all of which respond to targeted treatment.
How Multiple Causes Overlap
Hair loss rarely fits neatly into a single category. A woman with a genetic predisposition to thinning might notice it accelerate after pregnancy, during a period of high stress, or while dealing with low iron levels. Someone on blood pressure medication might assume the drug is the sole cause when a thyroid imbalance is also contributing. This overlap is one reason hair loss can be frustrating to pin down, and why a thorough evaluation that considers hormones, nutrition, medications, and family history is often more useful than guessing at a single cause.