Baldness happens primarily because a hormone called DHT (dihydrotestosterone) shrinks your hair follicles until they can no longer produce visible hair. This process, called miniaturization, is the driving force behind the most common type of hair loss, which affects about 1 in 5 men in their 20s and nearly half of all men over 40. But DHT isn’t the only cause. Autoimmune conditions, nutritional deficiencies, stress, and hormonal shifts can all trigger hair loss through entirely different pathways.
How Your Hair Grows and Sheds Normally
Every hair on your head cycles through three phases. The growth phase (anagen) lasts 3 to 10 years, during which the follicle actively produces hair. Then comes a brief transition phase (catagen) lasting 2 to 3 weeks, where the follicle detaches from its blood supply. Finally, the resting phase (telogen) lasts 3 to 4 months before the hair falls out and the cycle restarts.
Because each follicle operates on its own schedule, you lose between 50 and 150 hairs a day under normal conditions. That sounds like a lot, but your scalp holds roughly 100,000 follicles, so this daily turnover is invisible. A simple check: run your fingers through clean, dry hair and tug gently. One or two hairs is normal. If clumps come away in a single pull, something has disrupted the cycle.
DHT and Follicle Miniaturization
The most common reason for going bald is androgenetic alopecia, often called male or female pattern baldness. Your body converts testosterone into DHT, a more potent hormone. DHT binds to androgen receptors on hair follicles and, over time, causes them to shrink. Each growth cycle produces a thinner, shorter, lighter hair until the follicle eventually stops producing anything visible at all.
This doesn’t happen everywhere on your scalp equally. In men, DHT-sensitive follicles cluster at the temples and crown, which is why baldness typically starts with a receding hairline or a thinning spot on top. The follicles along the sides and back of the head are largely resistant to DHT, which is why even men with significant hair loss usually keep a horseshoe-shaped ring of hair.
The process is gradual. DHT shortens the growth phase from years down to months, so each successive hair has less time to grow before it falls out. The follicle itself physically shrinks with each cycle. By the time you notice thinning, miniaturization has likely been underway for years.
Genetics Determine Your Sensitivity
Whether DHT causes your follicles to shrink depends largely on your genes. The most well-established genetic factor is the AR gene, which provides the blueprint for androgen receptors. Variations in this gene can make your androgen receptors more easily activated by DHT, meaning your follicles respond more aggressively to the same hormone levels that leave someone else’s hair untouched.
The AR gene sits on the X chromosome, which is why people often say baldness comes from your mother’s side. There’s truth to that, but it’s not the full picture. Researchers suspect that variations in several other genes also contribute, making androgenetic alopecia a polygenic trait. You can inherit risk factors from both parents. This is why two brothers with the same mother can have very different hairlines, and why the inheritance pattern is difficult to predict cleanly from family history alone.
How Women Lose Hair Differently
Women experience pattern hair loss too, but the pattern and progression look different. Rather than a receding hairline, women typically notice thinning along the center part of their scalp, sometimes described as a Christmas tree pattern when viewed from above. The front hairline usually stays intact, and complete baldness is rare.
The causes in women are less clearly understood than in men. Aging, family history, and shifts in androgen levels all play roles. Hormonal changes during menopause, heavy menstrual bleeding (which can deplete iron stores), and certain medications can accelerate thinning. Because the mechanisms overlap but aren’t identical, treatments that work well for men don’t always translate directly to women.
Stress Can Push Hair Out Early
A condition called telogen effluvium occurs when a physical or emotional shock pushes a large number of follicles into the resting phase simultaneously. Instead of the usual staggered shedding, you lose hair in noticeable clumps, often while showering or brushing.
Common triggers include high fever, severe infections, childbirth, major surgery, significant psychological stress, thyroid disorders, crash diets low in protein, and certain medications. The hair loss typically shows up two to three months after the triggering event, which often makes it hard to connect cause and effect. The good news is that telogen effluvium is usually temporary. Once the underlying trigger is resolved, shedding slows over three to six months, and most people see full regrowth within six to eight months without any specific treatment.
When Your Immune System Attacks Follicles
Alopecia areata is an autoimmune condition where your immune system mistakenly targets hair follicles. Immune cells, primarily certain types of T cells, swarm the follicle in what pathologists describe as a “swarm of bees” pattern. This attack collapses the protective environment around the follicle and disrupts hair production.
The resulting hair loss looks different from pattern baldness. It typically appears as smooth, round bald patches that develop suddenly. In some people, it stays limited to a few patches. In others, it can progress to total scalp hair loss or even loss of all body hair. Alopecia areata can strike at any age and affects both men and women. Unlike pattern baldness, the follicles aren’t destroyed, which means regrowth is possible, though the course is unpredictable and relapses are common.
Nutritional Deficiencies That Thin Your Hair
Iron deficiency is the most common nutritional cause of hair loss. Your body prioritizes iron for essential functions like oxygen transport, and hair growth is one of the first things it cuts back on when stores run low. The key measure is ferritin, a protein that reflects your iron reserves. Standard lab reference ranges may list 15 to 30 ng/mL as “normal,” but research suggests that optimal hair growth requires ferritin levels of at least 70 ng/mL. Below 30 ng/mL, iron is highly likely contributing to hair thinning. Between 30 and 70, you may not have enough for your hair to thrive even though your bloodwork looks technically fine.
Vitamin D deficiency also shows up frequently in people experiencing hair loss, though the exact threshold isn’t as clearly defined. If you’re losing hair and can’t point to an obvious cause like family history or a recent stressful event, asking your doctor to check ferritin and vitamin D levels is a practical first step. These are simple, inexpensive blood tests, and the deficiencies are straightforward to correct.
What Can Actually Be Done About It
Two treatments have the strongest evidence behind them. Minoxidil, available over the counter as a liquid or foam applied to the scalp, works to some degree for about two-thirds of people who use it. It increases blood flow to follicles and appears to extend the growth phase, though the exact mechanism isn’t fully understood. It works for both men and women. Results take several months to appear, and the benefits reverse if you stop using it.
Finasteride is an oral prescription medication approved for male pattern hair loss. It works by blocking the conversion of testosterone to DHT, reducing the hormone levels that drive miniaturization. It’s effective for the majority of men who take it, both slowing further loss and, in many cases, producing some regrowth. Like minoxidil, the results are maintained only with continued use.
For hair loss driven by nutritional deficiencies or telogen effluvium, treatment focuses on correcting the underlying cause rather than the hair itself. Restoring iron stores to optimal levels, managing thyroid conditions, or simply allowing time to pass after a stressful event is often enough for hair to recover on its own. Alopecia areata, being autoimmune in nature, requires a different approach, with treatments aimed at calming the immune response around the follicles.