Thinning hair is most commonly caused by genetics, but hormonal changes, stress, nutritional deficiencies, medications, and physical damage to hair follicles can all play a role. Up to 80% of men and 50% of women develop some degree of genetic hair thinning by age 70, making it by far the leading cause. But because so many different factors can trigger or accelerate thinning, identifying your specific cause is the first step toward slowing it down.
Genetic Pattern Thinning
The most common form of hair loss is hereditary, and it works through a hormone called DHT (a byproduct of testosterone). DHT attaches to receptors on hair follicles and creates a compressing pressure that shrinks them over time. Each growth cycle, the follicle produces a slightly thinner, shorter strand. Eventually, thick terminal hairs are replaced by fine, nearly invisible ones. This process is called follicle miniaturization, and it’s why thinning from genetics tends to be gradual rather than sudden.
DHT also speeds up and shortens the active growth phase of each hair. Normally, a hair grows for two to six years before it naturally sheds. With genetic thinning, that window gets shorter with every cycle, so more follicles spend time resting rather than growing. The result is a slow reduction in overall hair density, typically starting at the temples and crown in men and as a widening part line in women.
Stress and Sudden Shedding
A major physical or emotional stressor can push a large number of hair follicles into their resting phase all at once. When those hairs finally shed two to three months later, you notice what feels like alarming amounts of hair falling out in the shower or on your pillow. This delayed timeline is why many people don’t connect the shedding to the event that caused it.
Known triggers include high fever, severe infections, major surgery, childbirth, and intense psychological stress. Crash diets that restrict protein can also set it off. The good news is that this type of shedding is usually temporary. Most cases resolve on their own within six months as the follicles re-enter their growth phase. Chronic, ongoing stress can extend the shedding, though, turning a short-term problem into a longer one.
Thyroid Problems
Both an underactive and overactive thyroid can cause thinning, but they affect hair differently. An underactive thyroid slows down cell division in the hair follicle, delaying the start of new growth cycles. Hair becomes coarse, dry, and brittle, and it grows slowly. You might also notice thinning at the outer edges of your eyebrows, which is a classic sign.
An overactive thyroid, somewhat counterintuitively, also causes hair loss. It accelerates hair development but weakens the hair shaft in the process, reducing its tensile strength. Hair becomes fine and silky but sheds more easily. The overactive thyroid also generates oxidative stress in follicle cells, damaging them from the inside. In both cases, treating the underlying thyroid condition typically allows hair to recover over several months.
Nutritional Deficiencies
Iron is the nutrient most closely linked to hair thinning. In one study, women experiencing diffuse hair loss had an average serum ferritin (the body’s iron storage marker) of about 15 ng/mL, compared to 25 ng/mL in women without hair loss. Your ferritin can technically fall within the “normal” lab range, which starts as low as 10 ng/mL, and still be low enough to affect your hair. This is one reason hair thinning sometimes gets dismissed when bloodwork comes back looking fine.
Vitamin D deficiency has also been associated with diffuse thinning. Other nutrients that support the hair growth cycle include zinc, biotin, and protein. If your diet is restrictive or you’ve recently lost a significant amount of weight, nutritional gaps are worth investigating as a contributing factor, even if they’re not the sole cause.
Medications That Trigger Thinning
Certain medications can cause thinning as a side effect, typically by pushing follicles into their resting phase prematurely. Common culprits include blood thinners, cholesterol-lowering drugs, high-dose vitamin A or retinoid-based medications, beta-blockers, some antidepressants, and anti-inflammatory drugs. Cancer treatments work differently: they damage rapidly dividing hair cells directly, which can cause more dramatic and rapid hair loss rather than gradual thinning.
Stopping or switching the medication usually reverses medication-related thinning, though regrowth can take several months. If you suspect a medication is involved, it’s worth reviewing your prescription timeline against when the thinning started, keeping in mind that two-to-three-month delay between cause and visible shedding. Hormonal contraceptives are another common trigger. Some women notice increased shedding after discontinuing birth control pills as hormone levels readjust.
Autoimmune Hair Loss
In autoimmune-related hair loss, the immune system mistakenly attacks hair follicles. Specific immune cells infiltrate the tissue around and inside the follicle, triggering cell death and hair loss. This type of thinning often looks different from other causes: rather than gradual overall thinning, it tends to produce distinct round patches of hair loss on the scalp.
Dermatologists can identify it through characteristic signs visible under magnification. Affected areas often show “exclamation-mark” hairs, where the strand is wider at the tip than at the root, and “black dots” where hairs have broken off right at the scalp surface. Some people experience a single episode that resolves on its own, while others deal with recurring or progressive loss. It can occur at any age and sometimes runs in families alongside other autoimmune conditions.
Tight Hairstyles and Physical Damage
Repeated tension on hair follicles from tight hairstyles can cause a specific type of thinning called traction alopecia. Braids, cornrows, tight ponytails, weaves, extensions, and dreadlocks are the most common causes. The risk increases significantly when tension is applied to hair that has already been weakened by chemical relaxers or heat styling, since these treatments compromise the hair shaft and make it more prone to breakage.
This type of thinning follows a two-phase pattern. In the early stages, the damage is reversible. You might notice hair loss along the hairline or wherever the pulling is most concentrated, but loosening the style allows regrowth. If the tension continues over months or years, though, it leads to inflammation, permanent scarring of the follicle, and irreversible stem cell damage. At that point, the follicle can no longer produce hair. The key is catching it early: receding around the hairline, tenderness at the roots, or small bumps on the scalp are all warning signs that a hairstyle is causing damage.
How to Tell What’s Normal
Shedding some hair every day is completely normal. Most people lose somewhere around 50 to 100 hairs daily, and this doesn’t lead to visible thinning because new hairs are growing in at roughly the same rate. When shedding consistently exceeds that range, or when the new hairs coming in are finer and shorter than what was lost, that’s when thinning becomes noticeable.
Because so many causes overlap, thinning hair often results from more than one factor working together. Someone with a genetic predisposition might not notice significant thinning until a period of stress, a thyroid shift, or a nutritional deficiency tips the balance. Identifying and addressing the reversible factors, like iron levels, stress, or damaging hairstyles, can make a meaningful difference even when genetics are part of the picture.