Male pattern baldness is caused by a hormone called DHT (dihydrotestosterone) shrinking hair follicles in people who are genetically susceptible to it. By age 35, about two-thirds of men will have some degree of noticeable hair loss, and by 50, roughly 85% will have significantly thinning hair. The process is gradual, often taking years or decades, but understanding the biology behind it explains why it follows such a predictable pattern.
How DHT Shrinks Hair Follicles
Testosterone circulates through your bloodstream, but it isn’t the direct cause of hair loss. An enzyme called 5-alpha reductase, concentrated in scalp and prostate cells, converts testosterone into DHT, a much more potent hormone. DHT then binds to receptors inside hair follicles on the top and front of the scalp, triggering a process called miniaturization: the follicles gradually produce thinner, shorter, lighter hairs with each growth cycle until they eventually stop producing visible hair altogether.
What makes this frustrating is that DHT actually stimulates hair growth in other parts of the body, like the beard and chest. The follicles on top of your head respond to DHT differently than follicles elsewhere because of their unique genetic programming. Hair on the sides and back of the head lacks the same receptor sensitivity, which is why those areas are typically spared even in advanced baldness.
The Genetic Component
The single most important gene in early-onset male pattern baldness is the androgen receptor gene, located on the X chromosome. Because sons inherit their X chromosome from their mother, the maternal side of the family plays an outsized role in determining hair loss risk. Research published in the American Journal of Human Genetics estimated that variation in this one gene accounts for about 46% of the risk for developing early hair loss.
This is why the old advice to “look at your mother’s father” has some truth to it, though it’s an oversimplification. The androgen receptor gene determines how sensitive your follicles are to DHT, but it can’t explain why fathers and sons often share similar balding patterns. That resemblance likely comes from additional genes on non-sex chromosomes that haven’t been fully identified yet. In other words, hair loss is polygenic: the X-linked androgen receptor gene is the biggest single player, but other inherited factors contribute to the overall picture.
What Happens to the Hair Growth Cycle
Each hair on your head cycles through a growth phase (anagen), a transition phase, and a resting phase (telogen) before falling out and being replaced. In a healthy scalp, the ratio of time spent growing versus resting is about 12 to 1. A single hair might grow for two to six years before entering a resting phase of a few months.
In male pattern baldness, DHT progressively shortens the growth phase and lengthens the resting phase. That 12-to-1 ratio can drop to 5 to 1 or lower. Each successive cycle produces a hair that is finer, shorter, and less pigmented than the one before it. Eventually the follicle produces only a tiny, nearly invisible vellus hair, the kind of fine peach fuzz you see on a child’s arm. The follicle is still technically alive, but functionally it has stopped contributing to visible hair coverage.
Inflammation and Scarring Around Follicles
DHT doesn’t just shorten the growth cycle. It also triggers low-grade, chronic inflammation around affected follicles. This inflammation activates cells that deposit collagen and other structural proteins around the follicle, forming a stiff “cuff” of scar-like tissue. Over time, this rigid layer chokes off nutrient exchange and the chemical signaling between different cell types that follicles need to regenerate properly.
This creates a self-reinforcing loop. DHT promotes inflammation, inflammation promotes scarring, and scarring physically compresses the follicle while cutting off the growth signals it depends on. As the tissue stiffens, the core structure of the follicle (the dermal papilla) shrinks in volume, growth phases get even shorter, and the follicle miniaturizes further. This scarring component helps explain why long-standing baldness becomes harder to reverse: it’s not just a hormonal signal problem but a structural one.
Common Patterns of Loss
Male pattern baldness doesn’t happen randomly. It follows recognizable patterns, most commonly starting with recession at the temples (creating an M-shaped hairline) or thinning at the crown. These two areas may progress independently for years before merging, eventually leaving a horseshoe-shaped ring of hair around the sides and back of the head.
Classification systems like the Hamilton-Norwood scale divide progression into stages. The earliest stages involve minor temple recession that many men don’t even notice. The middle stages show clear thinning on the crown or significant frontal recession. The most advanced stages involve complete loss of hair on the top of the scalp, with only the horseshoe fringe remaining. Not every man progresses through every stage. Some stabilize at moderate recession and never advance further, while others progress to extensive loss relatively quickly.
Why It Starts at Different Ages
Some men notice thinning in their early twenties, while others keep a full head of hair into their fifties. The timing depends on the specific combination of genetic variants you carry and how sensitive your follicles are to DHT. Men with shorter repeat sequences in the androgen receptor gene tend to develop hair loss earlier. Higher levels of 5-alpha reductase activity in the scalp, which means more local DHT production, also accelerate the process.
Stress, illness, nutritional deficiencies, and certain medications can cause temporary hair shedding that gets confused with male pattern baldness. The key difference is pattern: true androgenetic alopecia follows the predictable temple-and-crown distribution, while other causes of hair loss tend to be diffuse or patchy.
How Treatments Target the Cause
The two most established treatments work by interrupting the DHT pathway. One approach blocks the 5-alpha reductase enzyme, reducing the conversion of testosterone to DHT in scalp tissue. This lowers DHT levels at the follicle and can slow or partially reverse miniaturization in many men, particularly when started early. The other widely used treatment is a topical solution that improves blood flow to follicles and extends the growth phase, though it doesn’t address DHT directly.
Neither treatment can regrow hair on areas that have been completely bald for years, largely because of the fibrosis and structural damage described above. They work best on follicles that are miniaturized but still cycling. This is why earlier intervention tends to produce better results: the goal is to rescue follicles before they’re permanently compromised by scarring.