Hair thinning has dozens of possible causes, ranging from genetics and hormonal shifts to stress, nutritional gaps, and even the way you style your hair. By age 50, roughly 85% of men have noticeably thinning hair, and about a third of women will deal with some form of hair loss during their lifetime. Understanding what’s behind your thinning is the first step toward slowing or reversing it, because the right response depends entirely on the cause.
Genetic Pattern Thinning
The most common cause of hair thinning in both men and women is androgenetic alopecia, often called male- or female-pattern hair loss. In men, the process is driven by a hormone called DHT, a potent byproduct of testosterone. DHT binds to receptors inside the dermal papilla cells at the base of each hair follicle, triggering a chain reaction: the growth phase of the hair cycle shortens, the resting phase lengthens, and the follicle itself physically shrinks. Over time, thick terminal hairs are replaced by shorter, finer wisps that eventually stop growing altogether. By age 35, two-thirds of American men already have some visible thinning.
In women, the picture is less straightforward. Most women with the classic widening-part pattern of thinning actually have normal circulating androgen levels and no other hormonal symptoms like excess body hair or irregular periods. Pattern thinning has even been documented in women who completely lack androgen receptors. So while DHT clearly plays a role in some cases, genetics, local scalp chemistry, and factors researchers still don’t fully understand all contribute. Treatment typically starts with topical minoxidil, with anti-androgen options added when blood work shows elevated androgens or thinning is severe.
Stress and Telogen Effluvium
If your hair started thinning noticeably a few months after a major life event, you’re likely dealing with telogen effluvium. This is a temporary but alarming type of shedding that happens when a large percentage of your hair follicles are pushed prematurely into their resting phase. Instead of the normal 5 to 10 percent of hairs resting at any given time, a much larger share enters that phase simultaneously and then falls out weeks later.
The triggers are wide-ranging: high fever, severe infection, childbirth, major surgery, intense psychological stress, crash diets low in protein, and stopping birth control pills. The shedding typically begins two to three months after the triggering event, which is why people often don’t connect the cause to the hair loss. You’ll notice more hair in your brush, in the shower drain, and on your pillow.
The reassuring part: telogen effluvium usually lasts three to six months, and new growth appears in the affected areas once the shedding phase ends. No special treatment is needed beyond addressing the underlying trigger. If the stressor is ongoing, though, the thinning can persist.
Thyroid Problems
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause hair to stop growing or fall out. Thyroid hormones regulate metabolism in nearly every cell in your body, including the cells that build hair. When production is too high or too low, hair follicles can stall in their resting phase, leading to the same diffuse shedding pattern seen in telogen effluvium. You might notice thinning all over the scalp rather than in one specific area. If your thinning is accompanied by fatigue, weight changes, or feeling unusually cold or warm, a simple blood test can check your thyroid levels.
Iron and Nutritional Deficiencies
Low iron is one of the most underrecognized causes of hair thinning, particularly in women who menstruate, are pregnant, or follow restrictive diets. Research shows a striking connection: in one study, women with telogen effluvium had average ferritin levels (the protein that stores iron) of just 16.3 ng/mL, compared to 60.3 ng/mL in women without hair loss. Another study found that having ferritin at or below 30 ng/mL made a person 21 times more likely to experience this type of shedding.
Your ferritin can technically fall within a lab’s “normal” range and still be too low for optimal hair growth. Many dermatologists aim for ferritin above 40 or even 70 ng/mL when treating hair thinning.
Vitamin D deficiency also plays a role. Vitamin D receptors are present in both the keratinocytes and dermal papilla cells of hair follicles, and the active form of vitamin D helps prolong the growth phase and supports the proliferation of the cells that build hair. Widespread vitamin D deficiency has been documented in people experiencing hair loss, though researchers haven’t pinpointed an exact threshold below which thinning begins. Protein deficiency, zinc deficiency, and B-vitamin shortfalls can also contribute.
Hormonal Shifts During Menopause
Many women notice their hair becoming thinner and less dense as they approach or enter menopause. The reason is a shift in the hormonal balance. As estrogen and progesterone levels decline, androgens become relatively more active in hair follicles. This doesn’t mean androgen levels rise; it means the protective buffering effect of estrogen diminishes. The result looks similar to androgenetic alopecia: gradual thinning along the part line and crown, with the frontal hairline usually preserved. The transition can begin during perimenopause, years before periods stop entirely.
PCOS and Androgen Excess
Polycystic ovary syndrome is one of the most common hormonal conditions in women of reproductive age, and it can cause hair thinning through elevated androgen levels. Some women with significant androgen excess develop a thinning pattern at the crown that looks more like typical male-pattern loss. However, the relationship between PCOS, androgens, and hair loss is not as straightforward as you might expect. Many women with PCOS have excess body hair but no scalp thinning, and many women with scalp thinning have perfectly normal androgen levels. If your thinning is accompanied by irregular periods, acne, or facial hair growth, PCOS is worth investigating through blood work and an ultrasound.
Autoimmune Hair Loss
Alopecia areata is a condition where the immune system mistakenly identifies hair follicles as foreign invaders and attacks them. The classic sign is one or more smooth, round patches of hair loss, roughly the size of a quarter. But not every case looks like that. Diffuse alopecia areata causes overall thinning across the scalp rather than distinct patches, which can make it easy to confuse with other types of hair loss. In more extensive forms, it can progress to total scalp hair loss or even loss of all body hair. Alopecia areata can develop at any age, and it often comes and goes unpredictably.
Medications That Trigger Thinning
A surprising number of commonly prescribed medications can cause hair thinning as a side effect. The mechanism is usually the same as stress-related shedding: the drug pushes follicles into their resting phase prematurely. Onset typically occurs three weeks to three months after starting the medication.
Drug categories most commonly linked to thinning include:
- Blood thinners like heparin and warfarin
- Blood pressure medications, including certain ACE inhibitors and beta-blockers
- Retinoids used for acne and skin conditions
- Mood stabilizers and antidepressants
- Antifungal medications in the azole class
- Some NSAIDs (common over-the-counter pain relievers)
If your thinning started shortly after beginning a new medication, that connection is worth discussing with your prescriber. In most cases, hair recovers once the medication is stopped or switched.
Traction From Hairstyles
Traction alopecia is hair loss caused by repeated physical pulling on the follicles. It develops gradually, so it’s easy to dismiss early signs. You’ll first notice thinning in the areas where tension is greatest, often the hairline, temples, or edges. The highest-risk styles include tight ponytails, buns, braids, cornrows, locs, weaves, and extensions, especially when applied to chemically relaxed hair. Permanent waving and heavy hair accessories like beads or barrettes also contribute.
Early warning signs go beyond visible thinning. Pain, stinging, small bumps or pimples along the hairline, crusting, and a “tenting” appearance where the scalp lifts with the hair are all signals that too much tension is being applied. Caught early, the damage is reversible once the hairstyle changes. But if traction continues over months or years, the follicles scar over permanently and the hair won’t grow back.