The primary cause of balding in men is a hormone called dihydrotestosterone, or DHT, which shrinks hair follicles over time until they stop producing visible hair. Roughly 30% of men show signs of pattern baldness by their 30s, and the percentage climbs with each decade, reaching about 50% by age 50. While DHT and genetics drive the vast majority of male hair loss, other factors like stress, nutrition, thyroid problems, and even smoking can accelerate or compound the problem.
DHT and Follicle Miniaturization
Your body converts a portion of testosterone into DHT, a more potent hormone that binds to receptors on hair follicles across your scalp. When DHT locks onto these receptors, it triggers a process called miniaturization: the follicle gradually shrinks, produces thinner and shorter hairs with each growth cycle, and eventually stops producing hair altogether. This is why balding doesn’t happen overnight. It’s a slow progression where thick terminal hairs are replaced by fine, nearly invisible ones before the follicle shuts down entirely.
Not every follicle on your head responds to DHT the same way. The follicles along your temples and crown are far more sensitive to the hormone than those on the sides and back of your head. That’s why male pattern baldness follows such a recognizable shape: a receding hairline at the temples forming an M or V pattern, a thinning spot at the crown, and eventually a merging of the two. The follicles around the sides and back are largely resistant to DHT, which is why even men with advanced baldness retain hair in those areas.
The Role of Genetics
Your genes determine how sensitive your hair follicles are to DHT, and the most important gene sits on the X chromosome. The androgen receptor gene controls how your follicles respond to hormones, and specific variations of this gene are far more common in men who go bald. In one landmark study, a particular genetic marker in the androgen receptor gene appeared in 98% of young bald men compared to only 77% of men with no hair loss. Shorter repeat sequences within this gene were also more common in balding men, suggesting these variations cause the follicle to overreact to normal DHT levels.
Because the androgen receptor gene is on the X chromosome, you inherit it from your mother, which is where the old saying about looking at your mother’s father comes from. There’s truth to it, but it’s not the full picture. Male pattern baldness is polygenic, meaning multiple genes contribute. Research has identified risk factors on other chromosomes as well, so your father’s hair loss history matters too. If both sides of your family have significant balding, your odds are higher than if only one side does.
How Balding Progresses
Doctors use the Norwood Scale to describe the stages of male pattern baldness. In the earliest stage, there’s no meaningful hair loss. Stage 2 involves slight recession at the temples, often called a “mature hairline,” which is common and not necessarily a sign of progressive balding. Stage 3 is where clinically significant loss begins: the hairline recedes deeply at both temples into an M, U, or V shape, and the recessed areas are bare or nearly so. From there, the pattern advances through stages of increasing crown thinning and connection of the temple and crown bald spots, eventually leaving only a horseshoe of hair around the sides and back.
The pace varies enormously. Some men reach advanced stages in their 20s, while others thin slowly over decades. According to the International Society of Hair Restoration Surgery, the general rule of thumb is that 20% of men show pattern baldness in their 20s, 30% in their 30s, 40% in their 40s, and so on. By your 90s, the odds reach roughly 90%.
Stress and Telogen Effluvium
Stress-related hair loss works differently from pattern baldness. When your body experiences a major physical or emotional shock (severe illness, surgery, high fever, intense psychological stress), it can push a large percentage of hair follicles into their resting phase all at once. Two to three months later, those hairs fall out in clumps. This condition is called telogen effluvium, and it can be alarming because the shedding is sudden and diffuse rather than following the typical pattern of receding temples and crown thinning.
The good news is that telogen effluvium is almost always temporary. Hair typically grows back within three to six months without any treatment once the triggering event resolves. The bad news is that if you already have early pattern baldness, a bout of stress-related shedding can make the underlying thinning more noticeable and create the impression that you’ve lost ground quickly.
Thyroid Problems and Hormonal Imbalances
Both an overactive and underactive thyroid can cause hair loss in men. Thyroid hormones regulate metabolism throughout your body, including the growth cycle of your hair follicles. When those hormones are out of balance, follicles can stop growing new hair and shift into a prolonged resting phase. The result looks similar to stress-related shedding: diffuse thinning across the scalp rather than the temple-and-crown pattern of DHT-driven loss. Excessive shedding during washing, brushing, or styling is a common sign. Once thyroid levels are brought back to normal with treatment, hair loss from this cause is typically reversible.
Nutritional Deficiencies
Certain nutrient shortfalls can weaken hair or slow its growth, though they rarely cause the classic pattern baldness shape. Iron deficiency is one of the best-supported links: iron carries oxygen to hair follicles, and without adequate supply, growth slows. Vitamin D plays a role in creating the cells that form hair follicles, and low levels have been associated with increased shedding. Vitamin C helps your body absorb iron, so a deficiency in one can compound the other.
The evidence for other popular “hair vitamins” is more mixed. While biotin (vitamin B7) is heavily marketed for hair health, scientific support for supplementation in people who aren’t actually deficient is limited. And some nutrients can backfire: excessive vitamin A and selenium intake can actually increase hair loss. Taking megadoses of biotin can also interfere with blood test results, potentially masking thyroid problems that might be contributing to your hair loss in the first place.
Smoking and Scalp Blood Flow
Smoking damages hair in multiple ways. The toxic chemicals in tobacco smoke harm blood vessels, reducing circulation to the scalp. Hair follicles depend on blood flow for nutrients and waste removal, so impaired circulation can starve follicles and disrupt collagen production, leading to weaker, more brittle hair. Research has also shown that chemicals in cigarette smoke can damage the DNA of cells in hair follicles, potentially impairing their ability to grow hair normally.
There’s a compounding effect as well. A 2018 research review found that follicles in areas already affected by balding are particularly sensitive to oxidative stress, the kind of cellular damage that smoking promotes. So while smoking alone may not cause pattern baldness, it can accelerate it in men who are already genetically predisposed.
Scalp Conditions and Inflammation
Chronic scalp inflammation from conditions like seborrheic dermatitis (the condition behind persistent dandruff, redness, and flaking) can contribute to thinning in some cases. For most people with mild seborrheic dermatitis, any hair loss comes from scratching the irritated scalp rather than from the condition itself. In more severe cases, the inflammation can directly damage follicles enough to cause noticeable thinning. This is distinct from pattern baldness and usually improves when the underlying scalp condition is treated.
Alopecia Areata: A Different Type of Loss
Not all bald patches in men come from DHT. Alopecia areata is an autoimmune condition where the immune system attacks hair follicles, causing sudden, circular bald patches that can appear anywhere on the scalp. It looks and behaves nothing like pattern baldness. The patches come on quickly (sometimes preceded by itching or pain), have smooth, well-defined borders, and can overlap. Pattern baldness, by contrast, is gradual, predictable, and painless. If your hair loss appeared suddenly in round patches rather than slowly receding at the temples or thinning at the crown, the cause is likely autoimmune rather than hormonal.