Hair thinning happens when individual strands grow back finer than before, when follicles stop producing new hair altogether, or when more hairs than usual enter the shedding phase at once. Most people lose between 30 and 150 hairs from their scalp every day, which is normal turnover. Thinning becomes noticeable when that balance tips, either because of genetics, hormones, nutrition, stress, or damage to the follicles themselves.
Genetics and the Shrinking Follicle
The most common cause of hair thinning worldwide is androgenetic alopecia, often called pattern hair loss. It affects both men and women, though it shows up differently. In men, thinning typically starts at the temples and crown. In women, it tends to appear as a widening part or overall reduction in volume across the top of the scalp.
The mechanism centers on a hormone called DHT, a more potent form of testosterone. In people genetically predisposed to pattern hair loss, DHT binds to receptors in certain hair follicles and gradually shrinks them. Over time, the cell population inside the follicle’s base (the dermal papilla) decreases, physically reducing the follicle’s size. Each growth cycle produces a thinner, shorter, lighter strand until the follicle eventually stops producing visible hair at all. This process, called miniaturization, can take years or even decades to become obvious.
Genetics determine which follicles are sensitive to DHT and how early the process begins. For men, hair thickness often starts declining rapidly by age 30. Women tend to have their thickest hair around age 40, and 38% of women over 50 experience significant thinning.
Hormonal Shifts
Hormones regulate the hair growth cycle, so any major shift can trigger thinning. Menopause is one of the most common hormonal triggers for women. As estrogen levels drop, androgens (the same family of hormones that includes testosterone and DHT) become more active in hair follicles. The result is a gradual loss of density, particularly along the part line and at the temples.
Pregnancy, stopping or starting birth control, and polycystic ovary syndrome (PCOS) can all shift the hormonal environment enough to affect hair. In many of these cases, the thinning is temporary and reverses once hormone levels stabilize, though it can take six months to a year before new growth is visible.
Thyroid Problems Change Hair Texture and Volume
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause noticeable hair thinning. Thyroid hormones help regulate the growth cycle of hair follicles, so when levels are off, a higher percentage of hairs shift into the resting phase and eventually fall out. You might notice more hair in your brush or shower drain than usual.
Beyond shedding, thyroid imbalances also change how hair feels. Strands often become dry, coarse, and more prone to breakage. The thinning is usually diffuse, spread across the entire scalp rather than concentrated in one area. The good news is that once thyroid levels are corrected with treatment, hair typically recovers over several months.
Nutritional Deficiencies
Hair follicles are metabolically demanding, and they’re among the first structures to suffer when your body is short on key nutrients. Iron and vitamin D are two of the most well-documented deficiencies linked to thinning.
In one study of women with pattern hair loss, 54% were vitamin D deficient (below 20 ng/mL) and another 38% had insufficient levels (20 to 29 ng/mL). For iron stores, measured by a blood marker called ferritin, 40% had very low levels (below 30 µg/L) and an additional 34% fell into the low range (31 to 70 µg/L). That means nearly three-quarters of the women studied had suboptimal iron stores. Low ferritin is particularly common in women who menstruate, vegetarians, and people with digestive conditions that impair absorption.
Other nutrients that matter for hair health include zinc, biotin, and protein. Crash diets and extreme calorie restriction are a frequent, underrecognized cause of thinning because they deplete multiple nutrients at once. If your thinning started a few months after a major dietary change or weight loss effort, that timing is a strong clue.
Stress-Related Shedding
A condition called telogen effluvium is the classic stress-related cause of hair thinning. Normally, only about 15% of your scalp hair is in the shedding (telogen) phase at any given time. After a significant physical or emotional stressor, that number can jump to 30% or more. The stressor could be surgery, a serious illness, high fever, a major life event, or rapid weight loss.
The tricky part is the delay. Hair pushed into the resting phase by stress doesn’t fall out immediately. It takes two to three months before those hairs actually shed, which means the thinning often seems to come out of nowhere. You might suddenly lose clumps while washing or brushing your hair and have no idea what triggered it because the event happened months earlier.
Telogen effluvium is almost always temporary. Once the underlying stressor resolves, follicles gradually return to their normal cycle and regrowth begins. Full recovery typically takes six to nine months, though it can feel slow because new hairs need time to reach noticeable length.
Medications That Cause Thinning
A surprisingly long list of common medications can contribute to hair thinning as a side effect. The categories include blood pressure medications (beta-blockers, ACE inhibitors, diuretics), cholesterol-lowering drugs, antidepressants, blood thinners, epilepsy drugs, acne treatments containing vitamin A derivatives, birth control pills, hormone replacement therapy, NSAIDs, mood stabilizers, immune-suppressing drugs, and weight loss medications.
Chemotherapy drugs are the most well-known cause of medication-related hair loss, but they work through a different mechanism, actively targeting rapidly dividing cells. Most other medications cause a milder, more gradual thinning that resembles telogen effluvium. If you notice thinning within a few months of starting a new medication, that connection is worth discussing with your prescriber. In most cases, hair recovers after the medication is stopped or switched.
Physical Damage to Follicles
Repeated tension on hair follicles from tight hairstyles, braids, weaves, extensions, or ponytails can cause a type of thinning called traction alopecia. The earliest signs are usually thinning along the hairline or at the temples, wherever the pull is strongest. Caught early, this type of thinning is reversible by simply reducing tension on the hair.
Left unchecked, however, chronic tension can escalate to scarring alopecia, where inflammation destroys the stem cells and oil glands in the middle of the follicle. Scar tissue replaces the follicle, and hair can no longer regenerate in that spot. The affected skin often looks smooth and shiny. Some people also experience itching, tenderness, redness, or scaling before the hair loss becomes permanent. If you notice any of those symptoms around areas where your hair is thinning, it’s worth getting evaluated before scarring sets in.
Age-Related Thinning
Even without any medical condition, hair naturally thins with age. Individual strand diameter decreases over time, meaning each hair on your head becomes finer and weaker. This makes hair more susceptible to breakage and gives the overall appearance of less volume, even if you aren’t losing more hairs than usual.
The growth phase of hair also shortens with age. Younger follicles can sustain a growth phase lasting several years, producing long, thick strands. As follicles age, that growth window narrows, producing shorter hairs that don’t reach the same length or fullness. Combined with the hormonal and nutritional factors that accumulate over decades, age-related thinning is one of the most universal experiences of getting older. It’s not a single switch that flips but a gradual convergence of multiple biological shifts happening at the same time.
How to Tell What’s Causing Your Thinning
The pattern and timeline of your thinning are the two most useful clues. Diffuse thinning spread evenly across the scalp points toward telogen effluvium, thyroid issues, or nutritional deficiencies. Thinning concentrated at the crown, temples, or part line suggests androgenetic alopecia. Thinning at the hairline or wherever hairstyles pull tightest suggests traction. And thinning that started two to three months after a specific event, whether illness, surgery, medication change, or crash diet, strongly suggests a telogen effluvium trigger.
A basic blood panel checking thyroid function, ferritin, vitamin D, and hormone levels can rule in or out several of the most treatable causes. If the pattern is unclear or scarring is suspected, a dermatologist can examine the scalp more closely and, if needed, take a small biopsy to look at what’s happening at the follicle level. Many causes of thinning are reversible or manageable once identified, but the sooner you figure out which category you’re in, the more options you have.