Men lose hair primarily because of genetics and hormones. A hormone called DHT, which is derived from testosterone, gradually shrinks hair follicles until they can no longer produce visible hair. This process, called male pattern baldness, follows a predictable pattern: roughly 20% of men show signs in their 20s, 30% in their 30s, 40% in their 40s, and so on, climbing to about 90% of men in their 90s. But genetics isn’t the only driver. Stress, nutrition, medical conditions, and even smoking can all play a role.
How DHT Shrinks Your Hair Follicles
Testosterone itself doesn’t cause baldness. Your body converts some testosterone into a more potent hormone called DHT. When DHT binds to receptors on hair follicles, it triggers a process called miniaturization: the follicle physically constricts, making it harder for hair to grow. Follicles that once produced thick, healthy strands start producing thinner, more fragile hairs with weaker shafts that break or fall out easily.
This doesn’t happen overnight. Each hair on your head cycles through a growth phase (lasting two to six years), a transition phase, and a resting phase. DHT shortens the growth phase and extends the resting phase. Over multiple cycles, the follicle produces progressively thinner hair until it stops producing visible hair altogether. That’s why balding is gradual: you typically notice thinning at the temples or crown first, then the affected area slowly expands over years or decades.
The Genetics Behind Baldness
How sensitive your follicles are to DHT is largely determined by your genes, particularly variations in the androgen receptor gene. Research published in the Journal of Investigative Dermatology found that a specific genetic marker on this gene was present in 98% of young bald men, compared to only 77% of men without hair loss. Shorter repeat sequences within the gene were also more common in balding men, suggesting these variations make follicles more reactive to DHT.
The androgen receptor gene sits on the X chromosome, which men inherit from their mothers. This is why people often say baldness “comes from your mother’s side.” There’s truth to that, but it’s not the whole picture. Hair loss is polygenic, meaning multiple genes contribute. You can inherit hair loss risk from either parent. Looking at your maternal grandfather gives you a clue, but it’s not a guarantee in either direction.
Temporary Hair Loss From Stress and Illness
Not all hair loss in men is permanent. A condition called telogen effluvium causes widespread shedding that can be alarming but is usually reversible. It happens when a physical or emotional shock pushes a large number of hair follicles into their resting phase simultaneously. About two to three months after the triggering event, those hairs fall out in noticeable clumps.
Common triggers include high fever, severe infections, major surgery, significant psychological stress, thyroid problems, and crash diets that lack adequate protein. Certain medications can also cause it. The good news is that telogen effluvium typically resolves on its own within three to six months after the trigger is addressed. The hair grows back because the follicles themselves aren’t damaged.
Thyroid Problems and Diffuse Thinning
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can cause hair loss in men. Unlike male pattern baldness, which tends to follow a receding hairline or thinning crown, thyroid-related hair loss usually appears as diffuse thinning across the entire scalp. You may notice a general decrease in hair density and volume rather than specific bald patches.
Thyroid hormones play a direct role in the hair growth cycle. When your thyroid produces too many or too few hormones, hair follicles can stop growing and enter a prolonged resting phase. If you’re losing hair evenly across your head rather than in the classic male pattern, it’s worth having your thyroid levels checked. Once the underlying thyroid condition is treated, hair growth often resumes.
Nutritional Deficiencies That Affect Hair
Your hair follicles are among the fastest-dividing cells in your body, so they’re sensitive to nutritional shortfalls. Iron deficiency is the most well-documented nutritional cause of hair thinning. While standard lab ranges consider ferritin (your body’s iron storage protein) levels of 15 to 30 ng/mL as normal, dermatologists use higher thresholds for hair health. Levels below 30 ng/mL are highly likely to contribute to hair loss, and many specialists consider 70 ng/mL or above optimal for hair growth.
Zinc and other micronutrients appear in many hair growth supplements, but medical evidence doesn’t support supplementing unless you have a confirmed deficiency. For most men eating a varied diet, these deficiencies are uncommon. If you suspect nutrition is playing a role, a blood test is more useful than a supplement.
Scalp Conditions That Cause Shedding
Chronic scalp inflammation can directly damage hair follicles and obstruct normal growth. Seborrheic dermatitis, a fungal condition that causes flaky, itchy, irritated skin on the scalp, is one of the more common culprits. Excess oil production on the scalp creates an environment where a naturally occurring yeast called Malassezia overgrows, triggering inflammation that damages follicles.
The itching itself compounds the problem. Persistent scratching physically damages follicles and disrupts the growth cycle. Left untreated, the combination of inflammation, yeast overgrowth, and mechanical damage from scratching can cause noticeable thinning. Treating the underlying scalp condition usually allows hair to recover, though it takes time for follicles to return to normal production.
How Smoking Accelerates Hair Loss
Smoking damages hair follicles through two pathways. First, toxic chemicals in tobacco smoke may directly damage the DNA of cells inside hair follicles, impairing their ability to produce healthy hair. Second, smoking generates excessive free radical activity in the body, creating what’s known as oxidative stress. Research has found that cells in the hair follicles of balding scalps are particularly sensitive to this oxidative stress, meaning smoking may speed up the progression of genetic hair loss that would have happened more slowly otherwise.
Smoking also reduces blood flow to the scalp, cutting off some of the oxygen and nutrient delivery that follicles need. The combined effect can age your hair prematurely, leading to dryness, brittleness, and thinning earlier than your genetics alone would predict.
Treatment Options and What to Expect
Two main treatments have strong clinical evidence behind them. Minoxidil is a topical solution (or foam) applied directly to the scalp. It works by increasing blood flow to hair follicles, delivering more oxygen and growth factors, and extending the active growth phase of each hair. The 5% concentration produces the best results, with response rates around 82% in clinical studies. About half of men using the 5% formulation see significant regrowth (70% or more of lost hair returning), while others see moderate improvement or stabilization.
The other option works from the inside. Finasteride is a daily pill that blocks the conversion of testosterone to DHT. A 10-year Japanese study of 523 men found that over 91% experienced some degree of improvement, and more than 99% at least stopped losing additional hair. These are strong numbers, but the treatment requires ongoing use. Stopping either minoxidil or finasteride typically means hair loss resumes where it left off.
The earlier you start treatment, the better the results tend to be. Once a follicle has been miniaturized for years and has completely stopped producing hair, it becomes much harder to revive. Treatments work best at slowing active loss and regrowing recently thinned areas, not restoring hair that’s been gone for a long time.