Your Hair Is Thinning Everywhere, and Your Genes Explain Why.

The androgen receptor is the lock on your hair follicle cells. It determines how strongly those cells respond when DHT binds to them. Some people have highly sensitive locks. Others have locks that barely react. The difference comes down to the length of a single repeated DNA sequence called a CAG repeat.

The length of your CAG repeat directly controls your androgen receptor sensitivity. Shorter repeats mean higher sensitivity: your hair follicles respond intensely to DHT-mediated miniaturization. Longer repeats mean lower sensitivity: the same amount of DHT has less effect on your follicles. This is the single most heritable factor in male pattern baldness. If you carry shorter repeats, your follicles are simply more responsive to hormonal signals that trigger thinning.

What this feels like day to day: your hair doesn’t respond the way you expect it to. You can have normal testosterone levels, but if your androgen receptors are highly sensitive, your follicles are still shrinking. Conversely, you might have high testosterone but barely lose hair if your receptor sensitivity is low. That’s why some men with high hormone levels keep a full head of hair, while others with normal levels go bald.

SRD5A2 encodes the enzyme that converts testosterone into DHT. DHT is the hormone directly responsible for hair follicle miniaturization in androgenetic alopecia. The more efficiently your body converts testosterone to DHT, the more potent the signal telling your hair follicles to shrink.

The V89L variant in SRD5A2 affects how efficiently this conversion happens. Roughly 30 to 40% of the population carries a variant that alters DHT production. Some variants increase DHT production, accelerating hair loss; others decrease it slightly, offering modest protection. If you carry a variant that increases conversion efficiency, your follicles are bathed in higher DHT levels even if your testosterone is normal.

What this feels like: your hair has been thinning since your 20s or 30s, steadily and persistently. Every year you notice less density, especially on the crown or diffusely across the scalp. Your hormone levels come back normal when tested, but the thinning continues regardless. That’s because your body is converting available testosterone into DHT more efficiently than average.

Estrogen is protective for hair. It keeps follicles in the growth phase and slows the transition to shedding. The estrogen receptor, encoded by ESR1, is what allows your hair follicles to respond to circulating estrogen. If your receptors are less sensitive, that protective signal weakens.

The PvuII and XbaI variants in ESR1 affect receptor sensitivity. Approximately 40% of the population carries variants that reduce estrogen receptor function. These variants make hair follicles less responsive to estrogen’s growth-promoting effects, leaving them more vulnerable to DHT-driven miniaturization. This is why some women experience accelerated hair loss after hormonal shifts like pregnancy, perimenopause, or discontinuing hormonal birth control. Their estrogen levels drop, and their follicles are already less sensitive to what estrogen remains.

What this feels like: your hair thinning worsens around hormonal transitions. Pregnancy, stopping birth control, or approaching menopause correlates with visible shedding. Or you’ve always had fine hair and notice it gets progressively thinner over time. Your follicles aren’t getting the protective estrogen signal they need to stay in growth mode.

Hair growth depends on rapid cellular division and regeneration. Your hair follicles are among the fastest-dividing cells in your body. That process requires methylation, the addition of methyl groups to DNA. MTHFR is the enzyme that starts this methylation cycle by converting folate into its active form.

The C677T variant in MTHFR, carried by approximately 40% of people with European ancestry, reduces enzyme efficiency by 40 to 70%. That means your cells are struggling to complete the methylation cycle, slowing regeneration in hair follicles and making it harder for new hair to grow. You can eat plenty of folate and still be functionally depleted at the methylation level. Your cells simply can’t process the substrate quickly enough.

What this feels like: your hair is consistently thin, fine, and slow-growing. New hair comes in wispy rather than robust. You might have low ferritin or iron, or your skin and nails also grow slowly. You notice that supplements help, but generic B vitamins don’t seem to move the needle the way they should. Your body is stuck in a low-turnover state.

Vitamin D is essential for hair follicle cycling. It tells dormant follicles to wake up and enter the growth phase. The vitamin D receptor, encoded by VDR, is what allows your follicles to respond to circulating vitamin D. If your receptors are less sensitive, that activation signal is weak even when your vitamin D levels look adequate.

The BsmI and FokI variants in VDR affect receptor sensitivity and protein function. Roughly 30 to 50% of the population carries variants that reduce VDR efficiency. These variants impair follicle activation and cycling, meaning your hair spends less time growing and more time shedding. You can have a vitamin D level of 40 ng/mL, which is considered normal, and still have follicles that aren’t responding to the signal to grow.

What this feels like: your hair sheds consistently, especially in fall and winter when sunlight exposure drops. You take vitamin D supplements and your blood levels normalize, but the shedding doesn’t stop. Your follicles are stuck in a resting phase, unable to transition back into active growth even when vitamin D is available.

Iron is fundamental to hair growth. It’s a core component of hemoglobin that carries oxygen to hair follicles, and it’s essential for the enzymes that drive cellular division in the follicle matrix. HFE is the gene that regulates iron absorption in your intestines. Variants that affect HFE function can alter your iron status, either leading to iron accumulation or poor iron absorption depending on which variant you carry.

The C282Y and H63D variants in HFE are common, carried by roughly 10% of the population at heterozygous levels. If you carry variants affecting iron regulation, your follicles may not have adequate iron to fuel rapid cell division and hair growth. This doesn’t mean you have hemochromatosis; it means your follicles are struggling to get enough iron even when your serum iron looks borderline normal.

What this feels like: your hair is consistently thin and slow-growing. You’ve checked iron and ferritin, and they’re low-normal or low. You supplement iron and feel better, but if you stop, the fatigue and hair thinning return. Your follicles are dependent on stable, adequate iron availability to sustain growth.