The most common cause of male hair loss is genetics. Around 25% of men with hereditary hair loss start noticing it before age 21, two-thirds have visible thinning by 35, and roughly 85% have significantly thinner hair by 50. But genetics isn’t the only factor. Stress, medical conditions, medications, autoimmune disorders, and lifestyle habits can all play a role, sometimes on their own and sometimes layered on top of a genetic predisposition.
Genetics and Hormones: The Primary Driver
Male pattern baldness, known clinically as androgenetic alopecia, accounts for the vast majority of hair loss in men. The process centers on DHT, a hormone your body produces from testosterone. DHT binds to receptors on hair follicles in specific areas of the scalp, particularly the temples and crown. Over time, it causes those follicles to shrink. Each growth cycle produces a thinner, shorter hair until the follicle eventually stops producing visible hair altogether.
What determines whether DHT affects your follicles is largely genetic. The key gene identified so far is the AR gene, which controls how sensitive your hair follicles’ receptors are to DHT. Variations in this gene create receptors that respond more aggressively to normal levels of DHT, essentially making certain follicles more vulnerable to miniaturization even when your hormone levels are perfectly normal. The AR gene sits on the X chromosome, which is why maternal family history is often cited as a predictor. But researchers suspect multiple genes across different chromosomes contribute, so hair loss patterns from either side of the family can be relevant.
This process is gradual. It typically starts with a receding hairline or thinning at the crown and progresses over years or decades. The follicles on the sides and back of the head are largely resistant to DHT, which is why those areas tend to keep their hair even in advanced baldness.
Stress-Related Shedding
Stress triggers a distinct type of hair loss called telogen effluvium, which looks and feels very different from genetic thinning. Instead of a gradually receding hairline, you notice diffuse shedding across the entire scalp, often handfuls of hair coming out in the shower or on your pillow.
The mechanism is straightforward. When your body is under significant stress, cortisol levels rise as part of the fight-or-flight response. Sustained high cortisol pushes a large number of hair follicles from their active growth phase into a resting phase simultaneously. Your body also redirects nutrients and energy away from hair production toward vital organs like the brain, heart, and lungs. Two to three months after the stressful event, those resting hairs fall out in a noticeable wave.
Common triggers include major surgery, severe illness, rapid weight loss, emotional trauma, and high fevers. The good news is that telogen effluvium is almost always temporary. Once the underlying stressor resolves, follicles cycle back into growth mode and hair typically recovers within six to twelve months.
Thyroid Problems and Other Medical Conditions
Your thyroid gland produces hormones that directly influence the hair growth cycle. Thyroid hormones help prolong the active growth phase of hair follicles and stimulate the cells responsible for producing new hair. When thyroid levels are too low (hypothyroidism) or too high (hyperthyroidism), this growth phase shortens, and hair becomes thinner, drier, and more prone to falling out. The thinning is usually diffuse rather than patterned, affecting the whole scalp and sometimes the eyebrows as well.
Iron deficiency anemia, nutritional deficiencies (particularly zinc, biotin, and vitamin D), scalp infections, and chronic conditions like lupus can also contribute to hair loss. In many of these cases, treating the underlying condition allows hair to recover, though the timeline varies.
Medications That Cause Thinning
Several common drug classes can trigger hair shedding as a side effect. The hair loss typically appears weeks to months after starting the medication and reverses after stopping or reducing the dose.
- Retinoids: Used for severe acne and psoriasis, these are well-known triggers. In studies of acne patients, roughly 3 to 6% experienced hair loss depending on dose. At higher doses used for psoriasis, more than 63% of patients in one study reported thinning after six months.
- Mood stabilizers: Valproate, used for epilepsy and bipolar disorder, causes hair loss in an estimated 11% of users. Lithium has also been associated with hair thinning, though the evidence is less consistent.
- Antifungal medications: Certain antifungals taken orally can cause diffuse shedding. In one study, 79% of patients on long-term voriconazole developed scalp hair loss, with onset averaging about 75 days after starting treatment. Nearly all cases reversed after the drug was stopped or the dose reduced.
- Other common culprits: Blood thinners, beta-blockers, cholesterol-lowering drugs, and some antidepressants have all been linked to hair shedding in some users.
If you notice increased shedding after starting a new medication, the timing is a strong clue. Drug-induced hair loss typically begins one to three months after initiation and follows a diffuse pattern rather than the receding hairline of genetic loss.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where the immune system mistakenly attacks hair follicles. Certain white blood cells cluster around active follicles and shut down hair production. The result is distinctive: smooth, round patches of complete baldness that appear suddenly, often on the scalp but sometimes in the beard or elsewhere on the body.
Unlike genetic hair loss, alopecia areata can start at any age and doesn’t follow a predictable pattern. Some men experience a single episode with full regrowth. Others have recurring patches. In rare cases, it progresses to total scalp hair loss or total body hair loss. The condition affects roughly 2% of the population at some point in their lives, and having a family history of autoimmune conditions increases risk.
Lifestyle Factors
Smoking has a measurable impact on hair health. The toxins in cigarettes constrict blood vessels, reducing blood flow to the scalp. Hair follicles depend on a steady supply of nutrients and oxygen delivered through tiny blood vessels, and when that supply is restricted, the growth cycle is disrupted. Several studies have found that smokers are more likely to develop earlier and more severe hair loss compared to nonsmokers of the same age.
Poor nutrition plays a similar role. Hair is one of the fastest-growing tissues in the body, and it requires a steady supply of protein, iron, zinc, and B vitamins. Crash diets, very low-calorie eating patterns, and restrictive diets that cut out entire food groups can starve follicles of what they need. This type of thinning is usually reversible once nutrition improves, but recovery takes months because new hair grows at roughly half an inch per month.
How to Tell Which Type You Have
The pattern of hair loss is the most useful clue. A receding hairline or thinning concentrated at the crown, especially if it’s been gradual over years, strongly points to genetic hair loss. Sudden diffuse shedding across the whole scalp suggests telogen effluvium from stress, illness, or medication. Smooth round patches point toward alopecia areata.
Age matters too. Genetic hair loss can start in the late teens but typically becomes noticeable in the mid-20s to mid-30s. Telogen effluvium and autoimmune hair loss can strike at any age and often have an identifiable trigger if you look back two to three months before the shedding started. Multiple causes can also overlap. A man with a genetic predisposition to thinning may notice it accelerate during a period of high stress or after starting a new medication, making it harder to pinpoint a single factor.