The primary cause of baldness in men is a genetic sensitivity to a hormone called DHT, a potent form of testosterone that gradually shrinks hair follicles until they can no longer produce visible hair. Two-thirds of men show noticeable hair loss by age 35, and by 50, roughly 85% have significantly thinning hair. But genetics isn’t the only factor. Stress, nutrition, medications, and autoimmune conditions can all play a role.
How DHT Shrinks Your Hair Follicles
Male pattern baldness, the cause behind the vast majority of hair loss in men, comes down to how your hair follicles respond to dihydrotestosterone (DHT). Your body converts a portion of its testosterone into DHT, and in men with a genetic predisposition, DHT binds to receptors on scalp follicles and triggers them to miniaturize. The follicle shrinks, producing thinner, shorter, lighter hairs with each growth cycle until eventually it stops producing visible hair altogether.
This miniaturization was long thought to be a slow, gradual process spanning many hair cycles. More recent evidence suggests it can actually happen in large, abrupt steps. A follicle that was producing a thick terminal hair can suddenly shift to producing a fine, nearly invisible one in a single cycle. The mechanism appears to involve a sharp drop in the number of cells in the dermal papilla, the cluster of cells at the base of the follicle that controls hair size and growth duration. Fewer papilla cells means a smaller follicle, a thinner hair, and a shorter growth phase before the hair falls out. The encouraging flip side: this process can also reverse in a single cycle, which is why some treatments can produce noticeable regrowth relatively quickly.
The Genetics Behind Pattern Baldness
Your genes determine whether your follicles are sensitive to DHT in the first place. The most well-known genetic link sits on the X chromosome, which you inherit from your mother. This is why people often say to look at your maternal grandfather for clues about your hair future. That gene influences the androgen receptor, which is the docking station where DHT attaches to the follicle.
But the X chromosome isn’t the whole story. Multiple genes on other chromosomes also contribute, which means your father’s side of the family matters too. Pattern baldness is polygenic, meaning no single gene controls it. You can carry a strong genetic predisposition from one parent and a protective set of genes from the other, or vice versa. This is why brothers can have dramatically different hairlines despite sharing the same parents.
The Typical Pattern of Loss
Male pattern baldness follows a recognizable progression that doctors classify using the Norwood Scale, a seven-stage system. In the earliest stages, the hairline begins receding slightly at the temples. By stage 3, recession becomes more pronounced and thinning may appear at the crown. At stage 5, both areas of loss are clearly visible but still separated by a bridge of hair across the top of the head. By stage 6, that bridge thins out and the two bald areas merge. Stage 7, the most advanced, leaves hair only on the lower sides and back of the head.
Not every man progresses through all seven stages. Some stabilize at stage 3 or 4 and stay there for decades. Others move through the stages quickly, especially if hair loss starts in their late teens or early twenties. The earlier it begins, the more advanced it tends to become over a lifetime.
Stress and Sudden Shedding
Stress-related hair loss looks and behaves very differently from pattern baldness. A condition called telogen effluvium occurs when a physical or emotional shock pushes a large percentage of your hair follicles into the resting phase all at once. Normally, only about 10% of your hair is in this phase at any given time. After a major stressor, up to 70% of actively growing hairs can prematurely shift into the shedding phase.
The triggers are wide-ranging: high fever, severe infection, major surgery, significant psychological stress, thyroid disorders, and crash diets low in protein. The shedding typically shows up two to three months after the triggering event, which often makes it hard to connect cause and effect. The hair falls out diffusely across the entire scalp rather than in the receding-hairline pattern of genetic baldness. The good news is that telogen effluvium is almost always temporary. Once the underlying stressor resolves, hair regrowth follows within several months.
When the Immune System Attacks Follicles
Alopecia areata is a completely separate cause of hair loss driven by the immune system. In this condition, your body’s immune cells mistakenly identify hair follicles as foreign invaders and attack them. The result is hair falling out in small, round patches, usually about the size of a quarter, though larger areas can be affected. Some people experience a diffuse thinning pattern instead of distinct patches.
Alopecia areata can strike at any age and affects men and women equally. It’s distinct from pattern baldness in its appearance (patchy versus progressive recession), its cause (autoimmune versus hormonal), and its behavior (it can resolve spontaneously and then recur unpredictably). If you’re losing hair in well-defined patches rather than a gradual recession at the temples and crown, this is likely what’s happening.
Nutritional Gaps That Thin Your Hair
Several nutrient deficiencies are linked to hair thinning, though they rarely cause the dramatic baldness associated with genetics. Iron is essential for delivering oxygen to hair follicles. When iron levels drop low enough to cause deficiency, hair growth slows and shedding increases. Vitamin D plays a role in creating the cells that form new hair follicles, and low levels are associated with increased hair loss. The evidence for zinc is less consistent, with studies showing mixed results on whether supplementation helps.
The important caveat here is that taking supplements when you’re not actually deficient won’t prevent or reverse hair loss. Megadosing on biotin, for instance, has become popular but has no proven benefit for men with normal biotin levels. If you suspect a nutritional component to your hair loss, a blood test to check your actual levels is far more useful than guessing with supplements.
Medications That Can Cause Hair Loss
A surprisingly long list of common medications can trigger hair shedding as a side effect. The mechanism is similar to telogen effluvium: the drug disrupts the hair growth cycle and pushes follicles into the resting phase prematurely. Drug classes known to cause this include blood pressure medications (beta-blockers, ACE inhibitors, diuretics), cholesterol-lowering drugs, antidepressants, blood thinners, anti-seizure medications, NSAIDs like ibuprofen, thyroid medications, and acne treatments containing retinoids.
Chemotherapy is in a category of its own. Unlike other medications that cause gradual thinning, chemo drugs attack rapidly dividing cells throughout the body, and hair follicle cells are among the fastest-dividing cells you have. This is why chemotherapy often causes near-complete hair loss, though it’s almost always reversible once treatment ends. For non-chemo medications, the hair loss is typically less severe and resolves after switching drugs or adjusting dosages.
Why Only the Top of the Head
One of the most common questions about male baldness is why it spares the sides and back. The answer lies in the follicles themselves. Hair follicles on the sides and back of the scalp have a different genetic makeup than those on top. They either lack the androgen receptors that DHT binds to or express them at much lower levels. This is the same reason hair transplants work: follicles moved from the back of the head to the top retain their DHT resistance and continue growing in their new location.
This also explains why men with pattern baldness still have body hair and beard growth. DHT actually stimulates hair growth in those areas. The same hormone that shrinks scalp follicles promotes follicle activity on the face, chest, and arms. It’s the receptor configuration of each follicle, not the hormone level in your blood, that determines whether DHT helps or hurts hair growth in a given spot.