Hair falls out for dozens of reasons, ranging from everyday stress to chronic medical conditions. Losing between 50 and 150 strands a day is completely normal. When shedding consistently exceeds that range, or when you notice thinning patches, a widening part, or bald spots, something is disrupting your hair’s growth cycle. The cause is usually identifiable, and most types of hair loss are treatable or reversible once you know what’s behind it.
Genetics and Hormonal Sensitivity
The most common cause of hair loss worldwide is androgenetic alopecia, often called male-pattern or female-pattern hair loss. It happens when a hormone called DHT (a byproduct of testosterone) shrinks your hair follicles over time. Each growth cycle produces a thinner, shorter strand until the follicle eventually stops producing visible hair altogether. Genetics determine how sensitive your follicles are to DHT, which is why the condition runs in families.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it usually appears as a gradually widening part with overall thinning on top, while the hairline stays intact. The process is slow, unfolding over years or decades, which makes it easy to dismiss early on.
Stress, Illness, and Shock to the System
A condition called telogen effluvium is responsible for the dramatic, diffuse shedding that follows a major physical or emotional event. When your body goes through something intense, it pushes a large percentage of hair follicles into a resting phase all at once. Two to three months later, those hairs fall out in noticeable clumps, often in the shower or on your pillow.
Common triggers include high fever, severe infections, major surgery, childbirth, and significant psychological stress. Crash diets that lack adequate protein can do it too. The good news is that telogen effluvium usually resolves on its own within three to six months once the triggering event has passed. The hair that fell out is replaced by new growth, though it can take additional months before you notice the volume returning.
Thyroid Problems
Both an overactive and underactive thyroid can cause hair loss, and the pattern is distinctive. Rather than creating bald patches, thyroid-related shedding causes diffuse thinning across the entire scalp. You may notice your hair losing density and volume evenly rather than in one spot. The texture often changes too, with strands becoming dry, coarse, and brittle.
One telling sign is hair loss beyond the scalp. If your eyebrows are thinning (especially the outer third), or you’re losing eyelash, armpit, or body hair alongside scalp hair, a thyroid imbalance is worth investigating. A simple blood test can confirm it, and once thyroid levels are corrected with treatment, hair growth typically resumes.
Iron and Nutritional Deficiencies
Low iron is one of the most overlooked causes of hair shedding, particularly in women who menstruate. Your body needs iron to produce the proteins that build hair strands, and when stores run low, hair growth is one of the first things it deprioritizes. A study comparing women with and without hair loss found that those experiencing diffuse shedding had average ferritin (iron storage) levels around 15 ng/mL, compared to roughly 25 ng/mL in women without hair loss.
Clinicians often use a ferritin level of 40 ng/mL or lower as a threshold for iron deficiency, even though standard lab ranges may list lower numbers as “normal.” One study found that women with ferritin levels at or below 30 ng/mL were 21 times more likely to experience telogen effluvium than those with higher levels. That said, research on whether iron supplements alone reverse hair loss has been mixed, so it’s worth checking for other contributing factors as well.
Deficiencies in zinc, vitamin D, biotin, and protein can also contribute to thinning. Extreme or highly restrictive diets are a frequent culprit.
Autoimmune Hair Loss
Alopecia areata is a condition where the immune system mistakenly identifies hair follicles as foreign invaders and attacks them. The result is sudden, patchy hair loss, typically in smooth, round spots about the size of a quarter. Some people lose only a few patches. Others progress to losing all scalp hair (alopecia totalis) or all body hair (alopecia universalis).
Unlike pattern hair loss, alopecia areata can strike at any age, and the patches can appear and resolve unpredictably. The follicles aren’t destroyed, which means regrowth is possible, though recurrence is common. Treatments focus on calming the immune response in the affected area.
PCOS and Elevated Androgens
Polycystic ovary syndrome is a significant cause of hair thinning in younger women. PCOS elevates androgen hormones, which can trigger the same DHT-driven follicle shrinkage seen in genetic pattern hair loss. Research published in the Journal of the American Academy of Dermatology found that PCOS was prevalent among women under 30 experiencing female-pattern hair loss, and recommended hormonal evaluation for women in that group who develop early thinning.
If your hair loss coincides with irregular periods, acne, or excess facial or body hair, elevated androgens from PCOS may be the underlying cause. Addressing the hormonal imbalance can slow or stop the shedding.
Medications
A surprisingly long list of medications can trigger hair shedding as a side effect. The mechanism is usually telogen effluvium: the drug pushes follicles into a resting phase, and shedding starts a few months after beginning the medication. Drug classes known to cause this include blood thinners like heparin and warfarin, beta-blockers, calcium channel blockers, retinoids (used for acne and skin conditions), some antidepressants, and certain anti-inflammatory drugs.
Stopping or switching birth control pills is another well-documented trigger, since the hormonal shift can disrupt the hair cycle. If you notice increased shedding within a few months of starting or stopping a medication, that timing is worth mentioning to your prescriber. In most cases, the shedding resolves after the body adjusts or the medication changes.
Tight Hairstyles and Physical Damage
Traction alopecia results from hairstyles that pull on the hair roots over extended periods: tight ponytails, braids, buns, weaves, and extensions. The constant tension stresses the follicles and can lead to thinning or bald spots, especially along the hairline and temples. Some people experience “ponytail headaches” from the tension, which is itself a warning sign.
Caught early, traction alopecia is fully reversible. Loosening your hairstyles and reducing tension allows the follicles to recover. But if tight styling continues for months or years, repeated damage can cause scarring around the follicles. Once scarring occurs, those follicles can no longer produce hair, and the loss becomes permanent. If your hair hasn’t grown back several months after changing your styling habits, scarring may have already set in.
Scalp Conditions
Inflammatory scalp conditions like seborrheic dermatitis (a common cause of dandruff and flaking) can contribute to hair loss. Excess oil production on the scalp creates irritation and inflammation, which triggers intense itching. Scratching damages the follicles and disrupts normal hair growth. Psoriasis affecting the scalp works through a similar cycle of inflammation and scratching.
This type of hair loss is usually temporary. Once the underlying scalp condition is managed and the inflammation subsides, hair typically regrows in the affected areas. The key is treating the scalp condition rather than focusing on the hair loss itself.
How to Narrow Down Your Cause
The pattern and timing of your hair loss offer the strongest clues to its cause. Sudden, dramatic shedding that started two to three months after a stressful event points to telogen effluvium. Gradual thinning at the part line or crown over months and years suggests genetic pattern loss. Smooth, round bald patches that appeared quickly are characteristic of alopecia areata. Thinning concentrated around your hairline, temples, or wherever you wear tension may be traction alopecia.
Pay attention to what else is going on in your body. Changes in energy, weight, menstrual cycles, or skin can indicate hormonal or thyroid issues. A detailed look at your diet, stress levels, medications, and styling habits can rule out many of the most common triggers. Blood work checking thyroid function, iron and ferritin levels, and hormone panels can identify or eliminate several causes at once.