The most common cause of hair loss in men is genetics. Roughly 20% of men show noticeable hair loss in their 20s, 30% in their 30s, and 40% in their 40s, with the percentage continuing to climb each decade. But genetics isn’t the only driver. Stress, nutrition, medications, autoimmune conditions, and even smoking can all thin your hair or trigger sudden shedding.
Male Pattern Baldness and Genetics
Male pattern baldness, known clinically as androgenetic alopecia, accounts for the vast majority of hair loss in men. It’s driven by a hormone called dihydrotestosterone (DHT), which is derived from testosterone. DHT shrinks hair follicles over time, making each new growth cycle produce thinner, shorter strands until the follicle stops producing visible hair altogether.
The key genetic player identified so far is the AR gene, which provides instructions for building androgen receptors on your cells. Variations in this gene can make those receptors more sensitive to DHT than normal, meaning your hair follicles respond more aggressively to the hormone even at typical levels. The AR gene sits on the X chromosome, which men inherit from their mothers, and this is why people often look at the mother’s side of the family for clues. But the inheritance pattern is complex. Multiple genes and environmental factors are involved, so having a bald father or maternal grandfather doesn’t guarantee you’ll lose your hair, and plenty of men with no obvious family history still do.
The pattern itself is predictable enough that doctors use the Norwood scale, a 7-stage classification system, to describe it. Stage 1 shows little to no recession. By stage 3, thinning is visible at the temples or crown. Stage 7 represents hair loss across the entire top of the head, with only the lower sides and back remaining. Not every man progresses through all stages, and the speed of progression varies widely.
Stress-Related Hair Shedding
If you’ve noticed handfuls of hair coming out in the shower or on your pillow, especially after a difficult period in your life, you may be dealing with telogen effluvium. This is a temporary form of diffuse hair loss triggered by a shock to the body. Common triggers include high fever, severe infections, major surgery, significant psychological stress, and crash diets low in protein.
The hallmark of telogen effluvium is the delay. Hair doesn’t fall out during the stressful event itself. Instead, the stress pushes a large number of follicles into their resting phase all at once. Two to three months later, those resting hairs shed. This lag often makes it hard to connect the hair loss to its cause, because by the time you notice the shedding, the triggering event may feel like old news. Acute episodes typically resolve within six months as new hair grows in to replace what was lost.
Medications That Trigger Hair Loss
Several common drug classes can cause hair shedding as a side effect. Blood thinners are among the most well-documented culprits. Both older anticoagulants like heparin and warfarin and newer options can trigger shedding, typically starting three weeks to three months after you begin taking them. Retinoids (used for skin conditions and acne), certain blood pressure medications, mood stabilizers, and some antifungal drugs are also linked to temporary hair loss.
In most cases, medication-related hair loss follows the same telogen effluvium pattern: a delay of weeks to months, then diffuse shedding rather than the patchy or receding pattern of genetic hair loss. The hair usually regrows once the medication is stopped or switched, though that decision should be weighed against whatever the medication is treating.
Thyroid Problems and Nutritional Gaps
Both an overactive and underactive thyroid can trigger hair loss. Thyroid hormones regulate your metabolism at the cellular level, and hair follicles are sensitive to shifts in thyroid function. When levels are off, follicles can prematurely enter their resting phase, leading to thinning across the scalp rather than in one specific area.
Nutritional deficiencies play a role too, particularly low iron, zinc, and vitamin D. Hair follicles are among the most rapidly dividing cells in the body, so they’re among the first to suffer when key nutrients are in short supply. Men who follow restrictive diets, have absorption issues, or simply eat poorly may notice gradual thinning. A blood test can identify these deficiencies, and correcting them often slows or reverses the shedding.
Alopecia Areata
Alopecia areata is an autoimmune condition where the immune system mistakenly attacks hair follicles, treating them as foreign invaders like bacteria or viruses. The result is sudden, well-defined round patches of hair loss, often on the scalp but sometimes in the beard or elsewhere on the body. One characteristic sign is “exclamation point hairs,” short broken strands that are thicker at the top and taper toward the scalp.
Alopecia areata can appear at any age and affects men and women. In many cases the hair regrows on its own within months, though the condition can recur. Some people experience more extensive loss, and a small percentage lose all scalp hair or all body hair. It’s distinct from male pattern baldness both in its appearance (patchy versus receding) and its cause (immune-driven versus hormonal).
Scalp Infections and Inflammation
Fungal infections of the scalp, particularly tinea capitis (ringworm), can cause patches of hair loss along with redness and scaling. In its milder, non-inflammatory form, the hair loss is usually temporary. But the inflammatory form can produce painful, pus-filled patches called kerion, which are caused by the immune system’s reaction to the fungus. Kerion can lead to scarring and permanent hair loss in the affected area if not treated promptly.
Chronic inflammatory conditions like seborrheic dermatitis (the more severe cousin of dandruff) can also contribute to thinning over time. Persistent inflammation around the follicle disrupts normal hair growth, and scratching damaged, itchy skin compounds the problem. Treating the underlying inflammation usually allows hair to recover.
Smoking and Hair Loss
Smoking damages hair follicles through two mechanisms. First, the toxic chemicals in tobacco smoke increase free radical production in the body, creating oxidative stress that can damage the DNA of hair follicle cells and impair their ability to grow hair normally. Second, smoking harms circulation. It promotes plaque buildup in blood vessels, reducing the flow of nutrients and oxygen to the scalp. Hair follicles depend on a network of tiny blood vessels for nourishment, and compromised blood flow can accelerate thinning.
These effects compound whatever genetic predisposition you already have. A man with early-stage male pattern baldness who smokes may progress faster than he otherwise would, because the follicles that are already shrinking under the influence of DHT are also starved of nutrients and under oxidative attack.
How to Tell Which Type You Have
The pattern of hair loss is often the most revealing clue. A receding hairline and thinning at the crown that progresses gradually over years points to male pattern baldness. Sudden diffuse shedding, especially two to three months after an illness, surgery, or major stressor, suggests telogen effluvium. Smooth round patches appearing quickly are characteristic of alopecia areata. Scaly, red, or itchy patches with hair loss suggest a scalp condition like a fungal infection.
Many men have more than one factor at play. You might have a genetic predisposition that’s been accelerated by stress, poor nutrition, or a medication. A dermatologist can often distinguish between causes through a scalp examination and, if needed, blood work to check thyroid function, iron levels, and other markers. Identifying the cause matters because the treatments are very different: hormonal hair loss responds to DHT-blocking approaches, nutritional deficiencies respond to supplementation, autoimmune hair loss may need immune-modulating treatment, and stress-related shedding often resolves on its own once the trigger passes.