Men’s hair falls out primarily because of a hormone called DHT (dihydrotestosterone), which shrinks hair follicles until they can no longer produce visible hair. This process, called androgenetic alopecia or male pattern baldness, affects about 30% of white men by age 30 and 50% by age 50. But DHT isn’t the only culprit. Stress, nutritional gaps, scalp conditions, and autoimmune disorders can all trigger hair loss through entirely different mechanisms.
How DHT Shrinks Hair Follicles
Your body converts testosterone into DHT using an enzyme called 5-alpha reductase. DHT itself isn’t harmful in small amounts. At low concentrations, it actually supports hair growth. The problem starts when DHT accumulates at high levels around genetically sensitive follicles on the scalp, particularly along the hairline and crown.
At high concentrations, DHT blocks a critical growth signaling pathway inside the follicle. Normally, a protein called beta-catenin moves into the nucleus of hair cells and switches on the genes responsible for growth. DHT prevents that from happening. Without that signal, follicles gradually miniaturize: each growth cycle produces a thinner, shorter, lighter hair until eventually the follicle stops producing visible hair altogether.
This doesn’t happen overnight. Each hair on your head cycles through a growth phase lasting two to eight years, a brief two-week transition phase, and a resting phase of two to three months before shedding. DHT shortens the growth phase and lengthens the resting phase, so affected hairs spend less time growing and more time dormant. Over multiple cycles, the follicle weakens and the hair it produces becomes finer, until growth effectively stops.
Why Genetics Matter More Than You Think
Not every man with DHT in his system goes bald. The difference comes down to how your hair follicles respond to DHT, and that’s determined largely by genetics. The most influential gene is the androgen receptor gene, located on the X chromosome. Variations in this gene account for roughly 46% of the risk for early-onset male pattern baldness.
Because the androgen receptor gene sits on the X chromosome, you inherit it from your mother. This is why hair loss patterns often track the maternal side of the family. If your mother’s father was bald, your risk is higher. That said, the father-to-son resemblance in hair loss that many people notice comes from other genes on non-sex chromosomes, so both sides of the family contribute. The maternal line simply carries the single most important genetic factor.
Stress and Temporary Shedding
Not all hair loss in men is permanent. Telogen effluvium is a type of diffuse shedding triggered by physical or emotional stress. Rather than follicles shrinking over years, a large number of hairs are pushed into the resting phase all at once, then fall out together two to three months after the triggering event. Common triggers include surgery, high fever, significant weight loss, chronic illness, and severe psychological stress.
Nutritional deficiencies can do the same thing. Low iron stores, zinc deficiency, insufficient vitamin D, and severe caloric restriction have all been linked to telogen effluvium. Essential fatty acid deficiency typically causes shedding two to four months after intake drops too low. Certain medications, including beta-blockers, anticoagulants, antidepressants, and anticonvulsants, can also push hair into the resting phase, usually about 12 weeks after starting or changing a dose.
The reassuring part: acute telogen effluvium resolves on its own in about 95% of cases once the trigger is identified and removed. Hair typically regrows within six months.
Autoimmune Hair Loss
Alopecia areata is a different condition entirely. Here, the immune system mistakenly attacks hair follicles, causing inflammation that stops growth. The hallmark sign is one or more smooth, coin-sized bald patches that appear suddenly, usually on the scalp. Unlike pattern baldness, alopecia areata can strike at any age and doesn’t follow the typical receding hairline or thinning crown.
In more severe forms, it can progress to total scalp hair loss (alopecia totalis) or complete body hair loss (alopecia universalis), though these are rare. Genetics play a role here too: researchers have identified multiple genes involved in immune function that increase susceptibility. Some people also notice changes in their fingernails or toenails, like pitting or ridges.
Scalp Inflammation and Fungal Overgrowth
Chronic scalp conditions can accelerate hair loss or trigger shedding on their own. Seborrheic dermatitis, the condition behind persistent dandruff, is a particularly common contributor. It involves a yeast called Malassezia that naturally lives on your skin. In some people, this yeast breaks down scalp oils into fatty acids that trigger an inflammatory immune response, producing the characteristic yellow, greasy scales, redness, and itching.
That inflammation matters for your hair. Seborrheic dermatitis can cause telogen effluvium directly, and research shows it can also speed up the progression of androgenetic alopecia. Treating the scalp inflammation won’t reverse pattern baldness, but leaving it untreated can make existing hair loss worse.
Smoking and Oxidative Damage
Smoking accelerates hair loss through several pathways. Cigarette smoke generates free radicals that damage cell membranes and DNA in hair follicle cells. These damaged cells release inflammatory signals that actively inhibit hair growth. Smoking also constricts blood vessels, reducing nutrient delivery to the scalp.
Research on hair follicle cells from balding and non-balding scalp areas found that balding follicle cells already show signs of premature aging and elevated markers of oxidative stress. They grow more slowly in lab conditions and are more sensitive to environmental damage. Smoking compounds this vulnerability, essentially speeding up a process that’s already underway in genetically susceptible follicles.
What Treatment Looks Like
The two most established treatments for male pattern baldness work by targeting the mechanisms described above. One blocks the enzyme that converts testosterone to DHT, reducing DHT levels at the follicle. The other is a topical solution that stimulates blood flow to the scalp and extends the growth phase of the hair cycle. Used together, a large retrospective study of over 500 men found that 92% maintained their hair or improved over 12 months, with 57% showing clear visible improvement.
These treatments work best when started early, before follicles have fully miniaturized. Once a follicle has been dormant for years, it becomes much harder to revive. For temporary hair loss like telogen effluvium, treatment focuses on correcting the underlying trigger, whether that’s a nutritional deficiency, a medication change, or managing a chronic illness. Checking vitamin D, iron, and zinc levels is a reasonable first step for anyone experiencing unexplained diffuse thinning.