Hair thinning happens when individual strands become finer, follicles shrink, or more hairs enter the resting phase of their growth cycle than normal. The causes range from genetics and hormones to stress, nutritional gaps, and everyday styling habits. Understanding which type you’re dealing with is the first step toward slowing it down or reversing it.
How Hair Growth Works
Every hair on your head cycles through three phases: growth, transition, and rest. The growth phase lasts several years and determines how long your hair can get. Then comes a brief transition period of a few weeks, during which the follicle shrinks to about five-sixths of its normal diameter. Finally, the hair enters a resting phase where no new growth occurs. On a healthy scalp, about 10 to 15 percent of hairs are resting at any given time, and each resting hair can stay dormant for up to a year before falling out and being replaced.
Losing between 50 and 150 hairs a day is completely normal. The difference between everyday shedding and true thinning comes down to whether those lost hairs are being replaced by strands of the same thickness. When something disrupts the cycle, follicles either spend less time growing, produce thinner strands, or both.
Genetics and Hormones
The single most common cause of hair thinning is androgenetic alopecia, often called pattern hair loss. Up to 80 percent of men and 50 percent of women develop it by age 70. The mechanism centers on a hormone called DHT, a byproduct of testosterone. In people with a genetic sensitivity, DHT binds to receptors in certain hair follicles and triggers them to miniaturize. Each growth cycle gets shorter, and the follicle produces a thinner, wispier strand until eventually it stops producing visible hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, the part line gradually widens while the hairline usually stays intact. The process is slow, unfolding over years or decades, which is why many people don’t notice it until a significant amount of density is already gone.
Stress and Sudden Shedding
A condition called telogen effluvium is responsible for the dramatic, diffuse shedding people experience after a major physical or emotional event. Severe illness, high fever, surgery, difficult labor, significant blood loss, crash dieting, and intense emotional stress can all shock a large number of follicles into the resting phase simultaneously. The hair doesn’t fall out right away. Instead, shedding typically begins two to three months after the triggering event, which is why many people don’t connect the cause to the effect.
One study of 100 people with diffuse hair loss found that fever was the probable trigger in 33 percent, psychological stress in 30 percent, and systemic illness in 23 percent. The reassuring part is that telogen effluvium is usually self-limiting, resolving within about six months once the underlying trigger is addressed. During that window, though, the volume of hair coming out in your brush or shower drain can be alarming.
Thyroid Problems
Both an overactive and underactive thyroid can cause widespread hair thinning. Roughly 50 percent of people with hyperthyroidism and 33 percent of people with hypothyroidism experience noticeable hair loss. The two conditions affect hair differently, though. An underactive thyroid slows down cell division in the follicle, leading to coarse, dry, brittle hair that grows slowly. Loss of the outer third of the eyebrow is a classic sign. An overactive thyroid produces the opposite texture: hair becomes fine and silky but weaker, with reduced tensile strength and diffuse shedding across the scalp.
In hypothyroidism, follicles get stuck in the resting phase and are slow to restart growth. In hyperthyroidism, excess thyroid hormone ramps up oxidative stress in the follicle, damaging the structures that keep hair anchored and healthy. In both cases, hair density typically improves once thyroid levels are brought back to normal, though regrowth can take months.
PCOS and Elevated Androgens
Polycystic ovary syndrome is one of the most common hormonal causes of hair thinning in women of reproductive age. The condition drives up androgen levels, which can miniaturize scalp follicles in the same way DHT does in androgenetic alopecia. Insulin resistance, which frequently accompanies PCOS, amplifies the problem by further increasing androgen production. Women with PCOS often notice thinning at the top and front of the scalp, sometimes alongside excess hair growth on the face or body.
Nutritional Deficiencies
Iron is one of the best-studied nutrients in relation to hair thinning. Low iron stores, measured as serum ferritin, have a strong association with several types of non-scarring hair loss. In one study, 63 percent of women with alopecia had ferritin levels below 20 ng/mL, compared to a smaller proportion of women without hair loss. This held true across different types of thinning, from pattern loss to stress-related shedding. Some researchers suggest the threshold for hair-related problems may be even higher, in the range of 40 to 70 ng/mL, meaning your ferritin could technically be “normal” on a lab report but still low enough to affect your hair.
Vitamin D also plays a role. The vitamin D receptor on hair follicle cells is essential for initiating the growth phase of the hair cycle. Animal studies show that without a functioning vitamin D receptor, the hair cycle stalls and follicles fail to produce new growth. While the exact relationship in humans is still being refined, low vitamin D levels are frequently found in people experiencing unexplained hair thinning.
Vitamin A is a less intuitive culprit. While it’s essential for healthy skin and hair, excessive intake of vitamin A or its derivatives (common in supplements and certain acne treatments) can actually push follicles into the resting phase prematurely, causing diffuse shedding.
Medications
Certain categories of medication are well known for triggering hair thinning as a side effect. Chemotherapy drugs cause the most dramatic and rapid loss by directly damaging the fast-dividing cells in the hair matrix, leading to hair falling out during the active growth phase. This type of shedding begins within weeks of treatment and is usually reversible once treatment stops.
A broader range of medications causes a slower, more diffuse type of thinning by pushing follicles into the resting phase. Blood thinners, cholesterol-lowering drugs, high-dose vitamin A derivatives, and certain immune-modulating medications all fall into this category. The shedding typically appears a few months after starting the medication, mirroring the same delayed timeline as stress-related hair loss.
Traction and Styling Damage
Hairstyles that pull on the roots can cause a specific type of thinning called traction alopecia. Tight braids, cornrows, ponytails, weaves, extensions, dreadlocks, and even turbans can all generate enough sustained tension to damage follicles over time. The condition is particularly common among people who wear these styles consistently for months or years, and it also affects certain occupations: ballet dancers and military personnel are at elevated risk due to regulations requiring tightly bound hair.
Chemical relaxers and frequent heat styling compound the damage by weakening the hair shaft, making it more vulnerable to breakage under tension. Early signs include tenderness at the scalp, small bumps around hair follicles, broken hairs, and reduced density along the hairline or wherever tension is greatest. Caught early, traction alopecia is fully reversible by loosening hairstyles and reducing chemical processing. If the pulling continues, though, the damage becomes permanent as follicles scar over and lose the ability to produce hair.
How to Tell What Type You Have
The pattern of thinning offers strong clues. Diffuse thinning spread evenly across the scalp points toward telogen effluvium, thyroid issues, or nutritional deficiencies. Thinning concentrated at the temples, crown, or along the part line suggests hormonal or genetic causes. Thinning along the hairline or wherever hair is pulled tightest suggests traction damage.
Timing matters too. If you can trace the onset to roughly two to three months after a specific event (illness, surgery, starting a new medication, a period of extreme stress), telogen effluvium is the likely explanation. Gradual thinning that worsens over years without a clear trigger is more consistent with androgenetic alopecia.
A simple test clinicians use involves grasping a small section of about 20 to 60 hairs near the scalp and pulling gently. If more than 10 percent of those hairs come out, it indicates active shedding. When the test is positive all over the scalp, it suggests telogen effluvium. When it’s only positive in the area of visible thinning, pattern hair loss is more likely. You can try a rough version of this at home, but a dermatologist can examine the pulled hairs under magnification to identify the exact phase they were in when they fell, which narrows down the diagnosis considerably.