Most baldness comes down to genetics and hormones. A hormone called dihydrotestosterone (DHT) gradually shrinks hair follicles until they produce only fine, wispy strands or stop growing hair altogether. This process, called androgenetic alopecia, accounts for the vast majority of hair loss in both men and women. By age 35, two-thirds of American men have some degree of noticeable hair loss, and by 50, roughly 85% have significantly thinning hair.
But genetics isn’t the only reason people lose their hair. Autoimmune conditions, stress, nutritional deficiencies, and even certain hairstyles can trigger hair loss through entirely different mechanisms.
How DHT Shrinks Hair Follicles
Each strand of hair normally grows for two to six years, enters a resting phase for several months, then falls out and gets replaced by a new strand. Androgens, a family of hormones related to testosterone, help regulate this cycle. DHT is the most potent of these androgens, and in people genetically prone to baldness, it shortens the growth phase dramatically. The result: each new hair that grows back is thinner and shorter than the one before it. Over time, the follicle produces hair so fine it’s essentially invisible, and eventually it stops producing hair at all. This gradual shrinking is called miniaturization.
The genetic component centers on a gene called AR, which provides instructions for building androgen receptors on cells. Variations in this gene make androgen receptors more sensitive to DHT than normal, so even ordinary hormone levels can trigger excessive follicle shrinking. The AR gene sits on the X chromosome, which is why hair loss patterns often (though not always) track through the maternal side of the family. Other genes likely play a role too, but AR is the only one researchers have confirmed so far.
Why Women Lose Hair Differently
Women experience the same follicle miniaturization, but the pattern looks different. Instead of a receding hairline or a bald spot on the crown, women typically notice diffuse thinning across the top of the scalp while the frontal hairline stays intact. The exact hormonal triggers are less well understood than in men. Androgens play a role, but other factors, including fluctuations in estrogen during menopause, appear to contribute. Female pattern hair loss is also strongly genetic, though the inheritance pattern hasn’t been fully mapped.
Autoimmune Hair Loss
Alopecia areata is a fundamentally different kind of hair loss. Rather than hormones slowly miniaturizing follicles, the immune system attacks them directly. Hair follicles normally enjoy a kind of immune protection, a biological shield that keeps immune cells from targeting them. In alopecia areata, that shield collapses. Once the follicle’s proteins are exposed, the body treats them as foreign invaders.
Immune cells, primarily a type of white blood cell called CD8+ T cells, swarm around the exposed follicle in a pattern pathologists describe as a “swarm of bees.” These cells release inflammatory signals that shut down hair growth, causing hair to fall out in smooth, round patches. The condition affects an estimated 700,000 Americans and can range from a single coin-sized patch to complete loss of all body hair. Unlike pattern baldness, the follicles aren’t destroyed. They’re suppressed, which means regrowth is possible if the immune attack stops.
Three FDA-approved medications now exist specifically for severe alopecia areata. The most recent, approved in 2024, works by blocking the immune signaling pathway that drives the attack on follicles.
Stress-Related Shedding
A condition called telogen effluvium causes sudden, widespread hair shedding after a major physical or emotional stressor. Surgery, severe illness, childbirth, crash dieting, or intense psychological stress can all trigger it. The mechanism is straightforward: the stressor forces up to 70% of actively growing hairs to prematurely enter the resting phase all at once. Two to three months later, when those resting hairs reach the end of their cycle, they fall out in alarming quantities.
The delay between the stressor and the shedding is what makes telogen effluvium confusing. You might not connect a stressful event from months ago to the clumps of hair in your shower drain today. The good news is that acute telogen effluvium typically resolves within six months, and the hair grows back once the underlying trigger passes.
Nutritional Deficiencies That Thin Hair
Hair follicles are metabolically demanding. They need a steady supply of iron, vitamin D, and other nutrients to sustain their rapid growth cycle. Low iron stores are one of the most common nutritional contributors to hair thinning, particularly in women of childbearing age. Clinicians often look at ferritin, a protein that reflects how much iron your body has in reserve. Levels below about 10 to 15 nanograms per milliliter are strongly associated with diffuse hair loss in premenopausal women.
Low vitamin D is also frequently found in people experiencing hair loss, though the exact threshold that triggers shedding isn’t firmly established. Studies of women with diffuse hair loss have found average vitamin D levels well below the range considered sufficient. Thyroid dysfunction, both overactive and underactive, is another common culprit that disrupts the hair growth cycle. In all these cases, the hair loss is usually reversible once the deficiency or imbalance is corrected.
Hairstyles That Cause Permanent Damage
Traction alopecia happens when hairstyles pull on follicles with sustained tension. Tight cornrows, locs, braids, extensions, weaves on chemically relaxed hair, and ponytails or buns worn pulled back tightly can all cause it. Even the constant rubbing of a hat or headscarf over tightly pulled-back hair contributes. The early signs are small bumps on the scalp and short broken hairs along the hairline or part line.
What makes traction alopecia particularly important to recognize is that it’s preventable, but only up to a point. If the pulling continues long enough, the follicles scar over and the skin becomes smooth and shiny where hair once grew. At that stage, the loss is permanent. Caught early, loosening the hairstyle allows full regrowth. The American Academy of Dermatology recommends alternating between tighter and looser styles and watching for tenderness or redness at the hairline as warning signs.
Environmental and Lifestyle Factors
Air pollution is an emerging contributor to hair and scalp problems. Particulate matter, volatile organic compounds, and other pollutants landing on the scalp trigger oxidative stress, essentially overwhelming cells’ ability to neutralize harmful molecules. This damages the skin barrier, disrupts the scalp’s microbial balance, and promotes inflammation, all of which can weaken hair follicles over time. While pollution alone is unlikely to cause baldness, it may accelerate thinning in people already genetically susceptible.
Smoking has a similar effect. It restricts blood flow to the scalp and generates oxidative stress in follicle cells. Several studies have found higher rates of early hair loss in smokers compared to nonsmokers, independent of genetics.