Hair loss happens for dozens of reasons, but most cases trace back to a handful of common triggers: genetics, hormonal shifts, nutritional gaps, stress, medications, autoimmune conditions, and physical damage to the follicle. Losing 50 to 100 hairs a day is normal. When shedding consistently exceeds that range, or when hair visibly thins without growing back, something specific is usually driving it.
Genetics and Pattern Hair Loss
The single most common cause of hair loss is inherited pattern baldness, known medically as androgenetic alopecia. Up to 80% of men and 50% of women develop some degree of it by age 70. It runs in families on both sides, not just the mother’s side as the old myth suggests.
The mechanism works like this: a hormone called DHT (a potent form of testosterone) binds to receptors on genetically susceptible hair follicles. Each time a hair completes its growth cycle and falls out, the follicle shrinks slightly. The new hair comes in thinner and shorter than the one before it. Over many cycles, full-thickness hairs convert to fine, nearly invisible ones. The growth phase also shortens with each cycle, so hairs spend less time actively growing and more time resting. In men, this typically starts at the temples and crown. In women, it shows up as a widening part and overall thinning across the top of the scalp.
Stress and Telogen Effluvium
A major physical or emotional stressor can push large numbers of hair follicles into their resting phase all at once. This condition, called telogen effluvium, is the second most common cause of hair loss. The tricky part is the delay: hair doesn’t fall out during the stressful event. It falls out one to six months afterward, with three months being the average. That lag makes it hard to connect the shedding to its cause.
Common triggers include surgery, high fevers, significant weight loss, severe illness, emotional trauma, and crash diets. The shedding can be alarming because it’s diffuse, meaning hair comes out evenly across the scalp rather than in patches. The good news is that telogen effluvium is almost always temporary. Once the trigger resolves, hair typically regrows within six to twelve months.
Hormonal Changes
Pregnancy is a classic example. During pregnancy, elevated estrogen and progesterone keep hair follicles in their active growth phase longer than usual. Some follicles stay growing for the entire pregnancy. After delivery, all those follicles enter the resting phase at once, and the resulting shedding starts around two to four months postpartum. On average, shedding peaks around five months after delivery and resolves by about eight months, though it can linger up to a year.
Menopause triggers a different pattern. As estrogen declines, the relative influence of androgens increases, which can cause thinning similar to genetic pattern hair loss. Thyroid disorders affect hair growth in yet another way. Thyroid hormones directly influence how long follicles stay in their active growth phase. When thyroid levels drop too low (hypothyroidism) or spike too high (hyperthyroidism), the growth phase shortens and hair sheds more readily. Thyroid-related hair loss is diffuse and often affects the outer third of the eyebrows as well.
Iron and Nutritional Deficiencies
Iron deficiency is one of the most underdiagnosed causes of hair loss, especially in women. Here’s what makes it tricky: you don’t need to be anemic for low iron to affect your hair. Research published in Tzu Chi Medical Journal found that hair growth requires ferritin (the body’s iron storage protein) levels of at least 40 to 60 ng/mL. The standard cutoff for anemia corresponds to a ferritin of only about 5 ng/mL. That means your bloodwork could come back “normal” while your iron stores are still too low to support healthy hair growth.
If you suspect iron is a factor, ask specifically for a ferritin test, not just a complete blood count. Other nutritional shortfalls linked to hair loss include zinc, biotin, vitamin D, and protein deficiency. Extreme calorie restriction is a particularly effective way to trigger shedding because it combines multiple deficiencies at once.
Autoimmune Hair Loss
Alopecia areata causes sudden, round patches of hair loss that can appear on the scalp, beard, eyebrows, or anywhere on the body. It happens when the immune system mistakenly attacks hair follicles. Normally, hair follicles have a kind of immune privilege, meaning the body’s defenses leave them alone. In alopecia areata, that privilege breaks down. Immune cells swarm around and into the follicle, shutting down hair production.
The hallmark under a microscope is a dense cluster of immune cells surrounding the follicle base, sometimes described as a “swarm of bees” pattern. A specific type of immune cell, cytotoxic T cells, infiltrates the follicle itself and directly damages it. Natural killer cells also contribute by triggering cell death in affected follicles. Unlike pattern baldness, alopecia areata can strike at any age and often starts in childhood or early adulthood. The patches can regrow spontaneously, spread, or cycle between the two unpredictably.
Medications That Cause Shedding
Many common medications list hair loss as a side effect. The classes most frequently involved include blood thinners, retinoids (often prescribed for acne or skin conditions), certain antifungal medications, and mood stabilizers. Among blood thinners, both older options like heparin and warfarin and newer ones have alopecia listed as a potential adverse effect.
Blood pressure medications are less clear-cut. Hair loss linked to ACE inhibitors and beta-blockers has only appeared in isolated case reports, making them unlikely but not impossible culprits. If you notice increased shedding after starting a new medication, the timing is your best clue. Drug-induced shedding typically follows the same one-to-six-month delay as stress-related hair loss. Hair usually recovers after the medication is stopped or switched, though that decision should involve whoever prescribed it.
Physical Damage to the Follicle
Traction alopecia is hair loss caused by hairstyles that pull on the roots over time. Tight braids, cornrows, ponytails, weaves, extensions, dreadlocks, and even curlers can all cause it. The condition is also seen in ballet dancers, military personnel, and anyone whose occupation or cultural practice involves consistently tight styling. Chemical relaxers and heat styling compound the problem by weakening the hair shaft.
The earliest signs are redness around the follicles, small bumps along the hairline, and broken hairs in the areas under the most tension, usually the temples, the front hairline, and behind the ears. A characteristic “fringe sign,” a band of fine, miniaturized hairs along the hairline, is a red flag. If the pulling stops early, hair regrows. But years of sustained traction can permanently scar the follicle, at which point the loss becomes irreversible. Geometric or linear patterns of thinning that match the direction of tension are a strong indicator.
How to Tell What’s Causing Your Hair Loss
The pattern and timing of your shedding narrow down the cause significantly. Diffuse, all-over thinning that started a few months after a major event points to telogen effluvium. Gradual thinning concentrated at the crown or part line suggests genetic pattern loss. Sudden smooth patches suggest alopecia areata. Thinning along the hairline in areas of tension points to traction alopecia.
Consider what changed in the three to six months before the shedding started: a new medication, surgery, illness, crash diet, childbirth, or period of extreme stress. If nothing obvious comes to mind, bloodwork checking thyroid function, ferritin, vitamin D, and zinc levels can uncover hidden deficiencies. Multiple causes can overlap. It’s common, for instance, to have a genetic predisposition that only becomes noticeable after a stressor or nutritional dip tips the balance.